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Geomorphology, Quaternary Stratigraphy,

and Paleoecology of Central Texas

contributions by

Michael D. Blum

James F. Petersen

Rickard S. Toomey, III

Friends of the Pleistocene

CENTRAL TEXAS

South Central Cell, April 7-9, 1989

7th Annual Meeting

organized by

Stephen A. Hall and Thomas C. Gustavson

UL V. JiEINRlCIl PA .•

PHYSICAL GEOGRAPHY OF THE EASTERN EDWARDS PLATEAU

by

Michael D. Blum, Department of Geography, University of Texas at Austin

INTRODUCTION

The Edwards Plateau represents the southernmost extension of the Great Plains Physiographic Province. and is separated from the Gulf Coastal Plain to the east and south by the prominent Balcones Escarpment. The area covered by the 1989 Southcentral Friends of the Pleistocene field trip consists of three distinct physiographic zones within the eastern part of the Plateau: (1) the Llano area, where Enchanted Rock is located; (2) the dissected eastern margins of the Plateau, known locally as the "Hill Country", through which the Pedernales River flows; and (3) the less dissected Plateau surface where Halls' Cave is found (Figure 1). The following sections outline important characteristics of the modern climate, geologic selting, and vegetational mosaic. Readers interested in more thorough treatments.ot this area are directed to the references contained in each section.

Modern Climate The present climate of the eastern Edwards Plateau is classified as subtropical-subhumid to

semiarid and characterized by long, hot summers and mild, dry winters (Larkin and Bomar, 1983). Mean monthly temperatures in Fredericksburg range from a low of 90 C in January to a high of 280 C in August. Mean annual precipitation in Fredericksburg for the period 1951-1980 is 72.6 cm (Bomar, 1983), but there is a relatively steep precipitation gradient across the area, with annual means of 66 cm in westernmost Gillespie County and 81 cm in easternmost Blanco County (Larkin and Bomar, 1983). Precipitation maxi rna occur in both late spring and early fall, with a prirnary minimum during the winter rnonths and a secondary low during July and August (Figure 2). The average annual gross lake surface evaporation rate for Fredericksburg is 170 cm, and monthly values range from 7 cm in January to 25 cm in August. Average evaporation rates for all months are higher than the average precipitation (Larkin and Bomar, 1983).

Synoptic weather patterns responsible for the seasonal variations in precipitation are themselves highly variable. The eastem Edwards Plateau is dominated for much of the year by a meridional circulation pattern with southerly and southeasterly breezes, and the sources for moisture are maritime tropical airmasses that originate in the Gulf of Mexico and North Pacific. The dry winter months result from a more zonal or westerly flow, when frontal passage commonly occurs to frequently to permit advection of moist air from the Gulf of Mexico and North Pacific inland as far as the Edwards Plateau (Carr, 1967; Bomar, 1983). By contrast, during the summer months frontal storms rarely penetrate this far south, and the majority of precipitation results from localized, but often high intensity convectional storms within conditionally unstable maritime tropical airmasses. Abnormally dry summers are associated with the development of a strong and perSistent anticyclone over the Great Plains that blocks the influx of Gulf moisture (Carr, 1967; Bomar, 1983; Namias, 1982). During the late spring and early fall the majority of precipitation results from frontal storms, when less frequent migrations of cooler air masses from the north and west interact with unstable, moisture-laden winds from the Gulf and North Pacific: localized convective and topographically induced thunderstorms can be important rainfall producers during this time as well. The late summer to early fall peak also reflects the occasional inland movement of tropical easterly waves that can produce tremendous amounts of precipitation (Baker, 1975). In fact, when easterly waves reach tropical storm or hurricane status, then penetrate inland and come in contact with the irregular relief of the Edwards Plateau, some of the most intense rainfalls on record anywhere in the world have taken place (Baker, 1975; Caran and Baker, 1986).

During the period of historical monitoring, temperature has been relatively predictable on a year-to-year basis, but precipitation has varied a great deal. In fact, almost half of the years between 1931 and 1980 show annual precipitation values that depart from the mean by more than 25% (Carr, 1967;

Bomar, 1983; Figure 3). Year-to-year variations in the amount and spatial-temporal distribution of precipitation can be explained in terms of the same synoptic elements: (1) the frequency of frontal passage during the fall, winter, and spring months; (2) the intensity of development and persistance of the summertime high pressure cell over the Great Plains; and (3) the frequency of easterly waves, tropical storms, and hurricanes during the late summer and early fall.

Geologic Setting The Edwards Plateau Physiographic Region is the southernmost ex1enslon of the Great Plains

Physiographic Province. The Plateau is separated from the Gulf Coastal Plain to the east and south by the prominent Balcones Escarpment, a fault-line escarpment produced by a series of en echelon normal faults activated during the middle Miocene (Weeks, 1945; Caran et aI., 1982).

The Edwards Plateau surface represents an extensive tableland constructed of carbonate rocks deposited during the numerous marine transgressions that characterized the Cretaceous Period. The youngest of these are the Del Rio Clay and Buda Limestone of th& Upper Cretaceous Washita Division, now present only in the highest elevations of the Plateau (Barnes, 1981). Within the area covered by this field trip, the Plateau surface is underlain by the Ft. Terret and Segovia Members of the Edwards Limestone that make up the Lower Cretaceous Fredericksburg Division (Rose, 1972). Dissolution of the Edwards Limestone along joint surfaces, and dissolution and removal of interbedded evaporites, has produced many fine examples of karst landforms, undergound caverns such as Hall's Cave, and karst sedimentary deposits such as the collapse breccias exposed in many roadcuts (Kastning, 1983).

In the more dissected eastern margins of the Plateau, known locally as the "Hill Country", older Cretaceous carbonates and clastics are exposed as well. The Glen Rose Limestone, for example, consists of alternating beds of backwearing limestones and downwearing marls that make up the valley side walls of many streams in the Hill Country, including the Pedernales River, and is the uppermost rock unit preserved in the easternmost part of the Hill Country near the Balcones Escarpment (Stricklin et aI., 1971). In the Pedernales Valley, older Cretaceous clastics and carbonates are present, including the Hensel Sands which makes up the valley floor from Stonewall to west of Fredericksburg (Payne, 1982), and the Cow Creek Limestone, Hammett Shale, and Sycamore Sands which are exposed in the lowermost part of the drainage below the Pedernales Falls (Stricklin et aI., 1971). In the middle part of the Pedernales drainage, the channel transects an anticlinal positive relief feature in the pre-Cretaceous topography known as the San Marcos Arch, where Lower Paleozoic sandstones and carbonates, and PreCambrian igneous and metamorphic rocks are exposed (Stricklin et aI., 1971).

To the north and west of the Hill Country is the Llano area, which is presently a topographic basin, but records in its long and complex geologic history episodes of PreCambrian vulcanism, sedimentation, tectonic deformation and uplift, burial by Lower Paleozoic sediments, Upper Paleozoic sedimentation, tectonism, and erosion, burial by Cretaceous carbonates, then exhumation and erosion during the Cenozoic (McGehee, 1979; Peterson, this volume). The present landscape is circumscribed by an escarpment that represents the backwearing and removal of Cretaceous carbonates of the Edwards Plateau: pre-Cretaceous rocks exposed in the Llano area consist of a core of older PreCambrian metavolcanic and metasedimentary rocks and younger PreCambrian igneous intrusives, with a fringing cover of Paleozoic sandstones and carbonates (Barnes et al., 1972; McGehee, 1979; Peterson, this volume). The ubiquitous pre-Cretaceous faults pervade most of this area, and the resultant structures are reHected in the varying topography seen here (McGehee, 1979). Landforms common to the core of the Llano area, which Is underlain by the PreCambrian Town Mountain Granite, are the focus for day 1 of this field trip.

Modern Vegetation The modern vegetation of the eastern Edwards Plateau is a mosaic of species adapted to the

variability of the climatic regime, and represents an ecotone between plant communities that thrive in the more humid environments to the east and more arid lands to the west (Dunlap, 1983; Riskind and Diamond, 1988). Plant communities also strongly reflect geologic substrate, slope, and slope aspect, and vary significantly within the area covered by this field trip. Dominant arboreal species within the limestone-dominated terrain of the Hill Country and eastern Edwards Plateau consist of live oak (Quercus

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virginiana), shin oak (Q. sinuata). and Texas oak (Q. texana), with Honey mesquite (Prosopis g/andu/osa) , hackberry (Celtis spp.), and pecan (Carya iIIinoinensis) commonly found along stream bottoms. .. The shrub layer is dominated by the Ashe juniper (Juniperus ashel), and Texas persimmon (Diospyros te!lana). with the groundcover an admixture of cacti (e. g. the prickly pear, Opuntia spp.) as well as grasses that reflect the ecotonal nature of the plant community: tall grasses such as little bluestem (Schizachyrium scoparium), that are more common to the east, coexist with shorter grasses, such as various species of grama (e. g. Boute/oua curtipendu/a and B. rigidiseta) that are more common to the west and north (Gould, 1975). The deeply dissected easternmost Hill Country adjacent to the Balcones Escarpment, often referred to as the Balcones Canyonlands (Riskind and Diamond, 1986), has been termed the Live Oak-Ashe juniper Woodlands: further to the west, the overstory opens up and has been described as a Live Oak-Mesquite-Ashe juniper Parkland (McMahan et aI., 1984). The plant community in the Llano area is similar to that of the Hill Country and eastern Edwards Plateau, except for different species of oak, such as post oak and blackjack oak (Q. stellata and Q. marilandica), and the noticeable abscence of Ashe juniper on that part of the landscape dominated by acidic soils that develop-on Lower Paleozoic sandstones, and the PreCambrian igneous and metamorphic rocks (Riskind and Diarnond, 1988): vegetation of the Llano Basin has been described as a Live Oak-Mesquite Parkland by McMahan et al. (1984). According to Kier et al. (1977), the modern vegetation of the entire Edwards Plateau is a Juniper-Oak-Mesquite Savanna.

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100' 30' 31' r---------.",=-__ -=?lm--____ r-__ -,97'

EDWARDS

PLATEAU flail's Cave •

Kerrville

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N

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29° 30'

---- ---- ~ TEXAS /

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Figure 1, Location of stops for the 1989 Friends of the Pleistocene Field Trip, and the physiographic features mentioned in text. For more detail on geologic settings for each day of the trip, see the individual chapters in this guidebook.

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MONTHLY PRECIPITATION Fredeli;kSbJrg (16n·1973) Edwards Platea1J (1951·1960j

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J F M A M J J A SON 0

MONTH

75

70

65

60

55

50

eDWARDS PLATEAU ANNUAL PRECIPITATION [S year ruOfl"JJg r1'1eans)

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1930 1950 1970 1990

YEAR

Figure 2. Monthly distribution of precipitation for the town of Fredericksburg, and for the entire Edwards Plateau (left). Variability of the precipitation regime of the Edwards Plateau, as illustrated by 5 year running means covering the period 1931-1983 (right).

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Bibliography

Baker, V. R. (1975) Flood Hazards along the 8alcones Escarpment in Central Texas: Alternative Approaches to their Recognition, Mapping, and Management. Geological Circular 75-5. Bureau of Economic Geology, University of Texas at Austin. Austin, Texas. 22 p.

Barnes, V. E. (ed) (1972) Geology of the Llano Region and Austin Area. Guidebook 13. Bureau of Economic Geology. University of Texas. Austin, Texas.

Barnes, V. E. (1981) Geologic Atlas of Texas: The Llano Sheet. Bureau of Economic Geology. University of Texas. Austin, Texas.

Bomar, G. W. (1983) Texas Weather. University of Texas Press. Austin, Texas. 265 p.

Caran, S. C. and Baker, V. R. (1986) Flooding along the Balcones Escarpment, Central Texas. in Abbott, P. L.:. and Woodruff, C. M. (eds.) The 8alcones Escarpment: Geology, Hydrology, Ecology, and Social Development in Central Texas. Comet Reproduction Services. Sante Fe Springs, California. pp. 1-14.

Caran, S. C., Woodruff, C. M., and Thompson, E. J. (1982) Lineament analysis and inference of geologic structure: examples from the Balcones/Quachita trend of Texas. Transactions of the Gulf Coast Association of Geological Societies. v. 31, pp. 59-69.

Carr, J. T. (1967) Climate and Physiography of Texas .. Report 53, Texas Water Development Board. Austin, Texas. 27 p.

Dunlap, D. (1983) Quantitative Analysis of the Vegetation of the Texas Hill Country. Unpublished Thesis. University of Texas. Austin, Texas. 82 p.

Gould, F. W. (1975) Texas plants: A Checklist and Ecological Summary. Texas A &M University Press. College Station, Texas.

Kastning, E. H. (1983) Geomorphology and Hydrogeology of the Edwards Plateau Karst. Unpublished Dissertation, University of Texas at Austin. Austin, Texas. 656 p.

Kier, R. S., Garner, L. E., and Brown, L. F. (1977) Land Resources of Texas. Bureau of Economic Geology, University of Texas at Austin. Austin, Texas.

Larkin, T. J. and Bomar, G. W. (1983) Climatic Atlas of Texas. Texas Department of Water Resources. Austin, Texas. 151 p.

McGehee, R. V. (1979) PreCambrian Rocks of the Southeastern Llano Region, Texas. Geological Circular 79-3, Bureau of Economic Geology. University of Texas. Austin, Texas.

McMahan, C. A., Frye, R. G. and Brown, K. L. (1984) The Vegetation Types of Texas, Including Croplands. Texas Parks and Wildlife Department. Austin, Texas.

Namias, J. H. (1982) Anatomy of Great Plains protracted heat waves (especially the 1980 U. S. summer drought). Monthly Weather Review. v. 110, pp. 824-838.

Payne, J. (1982) Sedimentation and Pedogenesis in the Lower Cretaceous Hensel Sands. Unpublished Thesis, University of Texas at Austin. Austin, Texas. 136 p.

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Riskind, D.H and Diamond, D. D. (1988) An introduction to environments and vegetation. pp. 1-16 in Amos, B. B. and Gehlbach, F. R. (eds.) Edwards Plateau Vegetation: Plant Ecological Studies in Central Texas. Baylor UniversHy Press. Waco, Texas.

Rose, P. R. (1972) Edwards Group, Surface and Subsurface in Central Texas. Report of investigations 74. Bureau of Economic Geology, University of Texas at Austin. Austin, Texas. 198 p.

Stricklin, F. L., Smith, C. I., and Lozo, F. E. (1971) Stratigraphy of Lower Cretaceous Trinity deposits of Central Texas. RI 71. Bureau of Economic Geology. University of Texas. Austin, Texas. 37 p.

Weeks, A. W. (1945) Balcones, Luling, and Mexia Fault Zones in Texas. Bulletin of the American Association of Petroleum Geologists. v. 29, pp. 1733-1737.

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Friends of the Pleistocene-- South-Central Cell Seventh Annual Field Meetin : A ril 7-9, 1989

Granite Landforms of the Llano Uplift, with Emphasis on Enchanted Rock State Park, Texas

James F. Petersen Geography and Planning Southwest Texas State University San Marcos, Texas 78666

Texas "Hill Country" geomorphology is perhaps best known for stair-stepped limestone hills, the fluvially-dissected terrain of the Edwards Plateau. But a journey into the area that is deepest in the heart of Texas takes us to a much different landscape. This is the Central Mineral Region, noted for Precambrian granitic intrusions and older metamorphic rocks (Goldich 1941; Barnes, et al. 1972) exposed at the surface in the Llano Uplift, a broad structural high. Enchanted Rock exfoliation dome, located in a scenic state-protected natural area, is one of many interesting landforms formed on the "Texas Craton" (Flawn 1956). Following 140 years of private ownership, Enchanted Rock and 1,643 surrounding acres (665 hal were purchased by the Nature Conservancy and transferred to state administration in 1978.

This leg of the Friends of the Pleistocene meeting will be a radical departure from the norm-- from soft-rock to hard-rock, from "overburden" to basement, from stratigraphy to nonconformity, and from Pleistocene to Precambrian. Leave your picks, mattocks, entrenching tools, and shovels in the trunk, for the materials and landforms that we will examine will either be protected from excavation by state law, or will do serious damage to your tools of the Quaternary trade, or both. The Tertiary-Quaternary record in this region is dominantly erosional and other than fluvial depOSition, demonstration of landform ages is problematic. We will examine a host of granite landforms formed by the interaction of Precambrian lithology with mechanical and chemical weathering processes of debatable age and duration. As a replacement for digging, those who enjoy photography will (weather permitting) find an opportunity to photograph classic, world-class examples of granite landforms. For a preview, you can refer to Enchanted Rock State Natural Area: A Guide to the Landforms (Petersen 1988a), written for a popular audience. The field trip and this guide will examine the local geomorphology in more detail.

Stop 1. Fredericksburg City Cemetery. We will stop briefly at the Fredericksburg cemetery to examine the effects of rock weathering on locally-quarried granite, native limestone, and "imported" marble tombstones. We must be careful to observe the respect and care that a cemetery requires. Time spent examining the general weathering of tombstones and monument stones is instructive. Microfracturing and tiny spalling can be seen on granite monuments, particularly those exceeding about 75 years in age, indicating that small-scale exfoliation (micro-fracturing) processes operate rapidly.

Several old native limestone tombstones are in states of advanced weathering. It is unlikely that these native .carbonate markers experience the same conditions or rates of weathering that local rocks are subjected to, because horizontal-to-vertical rotation after quarrying has exposed the bedding planes to precipitation, and this has accelerated the weathering process. Compared to granite and native limestone, marble tombstones are in an intermediate state of decay, but show the effects of etching.

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Rates of Weathering Depression Formation: Twidale (1982) has found that within a few thousand years, weathering can produce pits on scree blocks. The process of pan formation in central Texas as suggested here operates at a rate that greatly exceeds this example. Of particular interest is a pair of weathering pits about 0.25 m by 0.35 m in a rough, unpolished, horizontal granite slab on a grave (dated 1887) in the cemetery (Petersen 1988b). Polishing inhibits weathering by reducing the amount of surface exposed to the elements, but this rough-hewn slab has a surface texture similar to natural granite surfaces found in central Texas. This suggests that the development of weathering depressions on the surface of Enchanted Rock (28 km away) can be an extremely rapid process.

Our excursion now takes us from Fredericksburg at an elevation of about 518 m (1700 ft ), through stair-stepped hills typical of those formed on the horizontally-layered carbonates of the Edwards Plateau. On Ranch Road 965 (Milam Street), we turn northward.

Stop 2. Roadside rest at Bear Mountain, former home of Balanced Rock. Bear Mountain Granite Quarry is on a hill between Enchanted Rock and Fredericksburg, 4 miles north of U.S. 290 on Ranch Road 965. The road here is at an elevation of approximately 549 m (1800 ft) and the top of Bear Mountain is about 575 m (1885 ft). Because the quarry is much closer to Fredericksburg, this granite outcrop probably saved Enchanted Rock from quarrying. The fractured hill is the former location of "Balanced Rock," a spheroidally-weathered corestone that until a couple of years ago was perched on top of the hill. In a 1986 act of vandalism, the rock was dynamited, and rolled off its pedestal. The trail to the top, once a natural tourist attraction, is closed to the public. Even though Balanced Rock is gone, the lonely rest area is a good spot to stop for a discussion of relevant components of the regional geology, derived from a number of sources including Barnes, et al. (1972), and several sections from the South-Central Decade of North American Geology volume (Amsbury 1988; Barnes 1988; Hutchinson 1988; Klier 1988).

Following the emplacement of Precambrian granitic plutons into older metamorphic rocks (about 1 billion years ago), accompanied by uplift, erosion removed several miles of rock (Barnes, et al. 1972). The result was a low-relief erosion surface, inundated by the sea during the Middle Cambrian. A variety of marine sedimentary rocks was deposited unconformably on this surface and deposition continued into the Pennsylvanian.

The Hickory Sandstone Member of the Middle Cambrian unconformably overlies Precambrian metamorphics and granites. The basal Hickory contains ventifacts and unweathered feldspars indicating that, " ... the marine transgression during which the Hickory was deposited crossed a dry, windy, and possibly cold, erosion surface developed on the Precambrian rocks, (Klier 1988: 352}." Paleotopographic relief, suggested by the basal unconformity, was about 244 m (800 ft), comparable to today (Klier 1988: 354).

Folding and faulting in the Late Paleozoic uplifted the region above sea level and deformed the Paleozoic sedimentary rocks. A major episode of erosion continued until the Cretaceous, when the sea again transgressed the land. Relief on the craton at that time has been estimated by Amsbury (1988: 373) as about 200 to 300 m (650 to 1000 feet ). The carbonate rocks, which today form the Edwards Plateau, were deposited during this time. During part of the Cretaceous, the Llano Uplift was expressed as islands, as many shallow near-shore environments are represented by the limestones of that time (Barnes, et al. 1972). At other times, the area probably was completely inundated. Marine deposition filled pre-Cretaceous valleys and buried hills.

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Bear Mountain has been interpreted as a pre-Cretaceous monadnock (erosion remnant, paleo-inselberg) by Virgil Barnes that "stands as high as the surface of the adjacent Edwards Plateau. About 180 feet (54 meters) of the monadnock has been exhumed; the amount that remains buried is unknown (Barnes 1988: 367)." But this site illustrates that the history of granite residuals in the Liana Uplift may reflect exhumation and multi-cyclic development, involving one or both of the previous episodes of denudation.

Uplift associated with the Balcones Fault Zone raised the Hill Country area above sea level. Subsequent erosion of the Cretaceous marine sediments exposed the Liana Uplift, because the cover of limestones was relatively thin over the Liana structural dome. Today, the structural uplift is a topographic basin, flanked by resistant cap rocks of the Edwards Plateau. Planation of the Liana Uplift has left many residuals standing as relief on the broad granitic pediment, including Enchanted Rock. Streams are slightly entrenched into the exhumed surface of the unconformity.

After our Bear Mountain stop, we continue north on Ranch Road 965, the next ten miles or so over Glen Rose Limestone-topped hills, and down into fluvially eroded valleys floored with Quaternary alluvium, or the Hensel Sand, the basal Cretaceous formation here. Finally we descend the edge of the erosional limestone escarpment that rims the eroded Liana topographic basin. Looking north, the angular limestone hills terminate, being replaced by more rounded forms-- pink outcrops of exfoliated and spheroidally-weathered granites. We are entering the Central Mineral Region, named for local mining, quarrying, and prospecting of graphite, soapstone, vermiculite and topaz (Barnes, et al. 1972). An interesting variety of granite landforms is found in the region. Granite hills and tors dot a landscape of moderate relief, generally ranging between elevations of 325 and 565 meters. Several massive granitic exfoliation domes form the horizon here, the largest one being Enchanted Rock. This is where we will spend the balance of the day.

Stop 3. Enchanted Rock State Natural Area. We will park near the trail head, not at the headquarters. Following the ubiquitous custom of government-protected parks, collection, excavation, and/or disturbance of natural features are forbidden activities. The following information may help you pass the time while we register the group at the park.

The name "Enchanted Rock" supposedly originated from legends and reported observations of strange lights and sounds apparently emanating from the top of the dome. A variety of explanations has been proposed concerning these phenomena. The gleaming appearance of feldspar cleavage planes reflecting light from the setting or rising sun gives the rock an attractive appearance, and the pink tone of the granite can seem to blend with the reddish tones of the sky at dusk. Tales of sounds coming from the rock involve creaking noises, usually at night. The most commonly cited explanation is the grinding of granite slabs due to thermal expansion or contraction, associated with rapid cooling that follows a warm day. Exfoliation sheet fragments and slabs on the rock are separated by narrow fractures and thermal expansion or contraction may cause grinding of a sheet over the bedrock, or against adjacent sheets. Although this explanation seems plausible, and there is evidence of compression between adjacent granite slabs on top of Enchanted Rock, the reports of sounds may be exaggerated (Petersen 1982). This author has camped many nights in the park, and has never heard any creaking sounds from the rock. Jim Wilson (personal communication 1981), former Park Superintendent, reported that during the three years that he was at the park, he made several unsuccessful attempts to personally witness the sounds. It is possible that the enchanted nature of the rock may be popular folklore or perhaps promotional "Ranger Lore," dating from the early years

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when the site was a privately-owned park. The sounds attributed to Enchanted Rock remain a mystery. Reverence for the rock is also attributed to the Comanche and earlier Indians of the area (for a review of local archaeology see Greer 1979).

Enchanted Rock Batholith: The pear-shaped surface exposure of the Enchanted Rock Batholith is about 24 kilometers (N-S) by 15 kilometers (E-W), and was mapped in detail by Hutchinson (1956). The batholith is composed of Town Mountain Granite, named after a locality, near the town of Llano, 29 km (18 mil northeast of the park. Dip of the contact between the Enchanted Rock Batholith and the surrounding Packsaddle Schist is steep and varies from about 60 to 85 degrees (Hutchinson 1956). Xenoliths of Packsaddle schist are found in the granite, and are usually parallel or sub-parallel to the planar flow structure (Hutchinson 1988). Aplitic or pegmatitic dikes and schlieren cut the surface in places. "Narrow veins of quartz and irregular pods of quartz enrichment occur on the northeast side of the dome near Sandy Creek (Hutchinson 1988: 371 )." The pluton formed about 1 billion years ago (Zartmann 1964), and is zoned in a transitional­concentric pattern of differing granitic porphyries (Hutchinson 1956). These lithologic variations are reflected in the topography as granite hills tend to form along the batholithic perimeter. Figure 1., from Hutchinson (1956), illustrates the lithologic zonation of the Enchanted Rock Batholith.

EXPlANAnON

SIDJ.'ENTARY ROCxS

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Figure 1. Geology of Enchanted Rock Batholith, Hutchinson (1956).

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Lithology: The Town Mountain Granite is a pink, coarsely crystalline, porphyritic rock consisting mainly of microcline plagioclase, quartz, and biotite. Barnes, Dawson, and Parkinson (1947) in a study of building stones in central Texas, conducted a detailed examination of the Town Mountain granite at Enchanted Rock, finding the following composition: 33% quartz, 34% microcline, 25% albite, 4% anorthite, and about 3% biotite. The distinctive color that dominates the terrain in wide areas of the Central Mineral Region results from an abundance of large salmon-colored microcline phenocrysts. Vertical planar flow structures (vertically-aligned microcline phenocrysts) strike N40-N50 E (Hutchinson 1988: 369). The bright color and durability of this granite make it an attractive building stone that has been quarried in great quantities (Barnes, Dawson, & Parkinson 1947). The largest quarrying operation, the Granite Mountain Quarry, is located near Marble Falls, Texas, about 35 miles (56 km) to the east. In the 1800s the owners donated granite building stone to build the Texas State Capitol in Austin.

Enchanted Rock Exfoliation Dome, Bornhardt Geomorphology: Enchanted Rock, a 136 meter high exfoliation dome (556 m elevation), is one of several granitic hills that project above the pedimented surface of the Enchanted Rock Batholith. Enchanted Rock has a convex slope form that steepens on the flanks, in a domal shape that has apparently resulted from sheeting processes. Granular disintegration and hydration of certain minerals (particularly biotite) have also contributed to the roundness of the dome. The topographic expression of Enchanted Rock is that of a classic born hardt, with a rounded form of exposed granite separated by valleys formed in jointed bedrock. Bornhardts were named after a German explorer who first scientifically described bald, steep-sided crystalline domes in Africa (Twidale 1982: 8). The residual monoliths in the park trend northeast to southwest on the edge of the pear-shaped exposure of the batholith, which is surrounded by schistose metamorphic rocks (Packsaddle Schist). A topographic map of the park area shows this alignment and the division of the massif into smaller outcrops, of which Enchanted Rock is the largest and highest. Variation in fracture density is an important control of the hill-valley forms. Sandy Creek, the wash at the base of Enchanted Rock, generally follows a curved path between metamorphic and granitic rocks, and/or the sheared, cataclastic zone that forms the contact on the perimeter of the batholith. Entrenched in this zone of weakness, the creek will migrate laterally (slightly) as it erodes down the dip of the contact, much like homoclinal shifting. A cross­section of the Enchanted Rock area is shown in Figure 2., from Hutchinson (1956).

~--- ~ ~

Figure 2. Geologic section through Enchanted Rock, looking northeast, 2-x vertical exaggeration (Hutchins9n 1956).

Though sometimes referred to as an inselberg, this term does not specifically apply, as Enchanted Rock is one of several adjacent domes and not an isolated hill. Enchanted Rock has a surface area of about 198 hectares (80 acres), but it is widely reported (erroneously) to be larger-- either one mile (1.6 km) in diameter, or one square mile in

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area (1581 hectares). The Texas Department of Parks and Wildlife reports that Enchanted Rock is the second largest exfoliation dome in the United States, second only to Stone Mountain, Georgia. A minor point perhaps, but Strahler (1969: 399) considers Stone Mountain to be something other than a true exfoliation dome, because it rises above a surrounding area of weaker metamorphic rocks. .

Sheet Structure: The presence of many exfoliation slabs on the surface suggests that unloading is the dominant control of the form of Enchanted Rock. Broken remnants of spalled sheets remain on the dome, displaying a jigsaw puzzle-like fit, that indicates the former extent of full sheets. The exfoliation slabs on Enchanted Rock range in thickness from less than 2.5 cm to over 3.5 m. The process of sheeting continues to modify the surface of the rock today. Near the summit on the east side of the dome, thin sheeting is particularly well-developed. Many sheets have "popped up" much like a blister, leaving a thin air space between the slab and the bedrock of the dome. Most of these sheets are less than 2 cm thick and have a bit of spring when walked upon. Footsteps or tapping on these slabs produces a hollow sound. In other places this thin crust has been broken, exposing the fresh and unweathered granite bedrock. Remnants of thick exfoliation slabs are best seen on the west side of the dome, along with evidence of down-slope sliding of . many places. From this side one can observe excellent examples of sheeting on Little Rock, a smaller dome immediately to the southwest.

There are few talus slopes at the base of the dome. Blank (1951 a) interpreted this to indicate that the rate of down-slope creep of the slabs was exceeded by the rate of weathering. A coarse crystalline microrelief of microcline phenocrysts studs the surface, and this roughness creates enough friction to impede the down-slope movement of slabs. However, in some areas, there is clear evidence of sliding of exfoliated blocks and slabs. Only beneath the steep northwest side is there any appreciable accumulation of talus blocks. This face of Enchanted Rock is much steeper than the other sides, due to joint spacing on that side, rather than any slope asymmetry caused by differential weathering on a slope having a north-facing orientation. Other domes in the area display no comparable steepness on slopes having a northwest aspect and major joints parallel the steep side of ER. The northwest slope is a popular place for rock climbing, and several routes up the steep face have been rated in terms of difficulty by local rock climbing groups.

Tors, Corestones, and Spheroidal Weathering: Twidale (1982: 16) regards "tor" as a difficult term to define, due to widespread variation in usage in the literature, but he prefers an operational definition suggested by Linton (1952, in Twidale 1982). "Tor: a bare rock outcrop, usually of monumental form, about the size of a house, commonly bounded by near-vertical fractures, and boldly fissured by widely-spaced joints." The lower flanks of Enchanted Rock are littered with residual boulders exhibiting varying degrees of spheroidal weathering. Stacks of corestones, exposed by stripping of weathered granite, stand in piles to form tors. Residual boulders are common granitic forms, perhaps the " ... most numerous and widely distributed of the positive relief forms developed on granite (Twidale 1 982: 89)." Joint orientation and spacing affect the shape of corestones and ultimately, the exposed tors. If the sets of joints are widely-spaced horizontally, and closely-spaced vertically, tall, narrow tors will form. Short and wide tors will form where the joints are shallow and widely-spaced.

The production of granite relief in the form of either boulders or hills is a two-stage process-- first, weathering and grus production, followed by erosional stripping (Linton 1955, 1964). These two processes may occur separately or simultaneously, but do not

6

necessarily operate at the same rate. "If erosion out paces weathering ... corestones are exposed as boulders, but if weathering proceeds more rapidly than erosion, all corestones located in the near surface zone will be reduced to grus (Twidale 1982: 94-95)." Figure 3. shows the development of tors.

Figure 3. The two-stage development of tors (Twidale, Campbell, Foale 1987).

Twidale (1976,1982: 125) contends that bornhardts have been found to be " ... characteristically developed in multicyclic landscapes, that is, in landscapes in which remnants of paleoplains preserved high in the relief indicate former phases of baselevelling, subsequent relative uplift and stream incision." The role of multi-cyclic weathering and stripping cannot be ignored in the ER area, because of the former covers of Paleozoic and then Cretaceous sedimentary rocks, on a paleo-plain (unconformable surface). This surface has been exposed to several episodes of exposure to weathering processes, stripping, and exhumation. The surfaces of the unconformities are sharp and unweathered in some locations, and weathered in others (Barnes 1988).

weathering front

Figure 4. The two-stage development of inselbergs (Twidale, Campbell, Foale 1987).

7

Cavernous Weathering: The edges of many slab remnants have been modified by basal weathering which has produced small overhangs and pedestal rocks. Down-slope migration of groundwater along exfoliation joints contributes to greater wetting frequency and weathering is accelerated in the zone between the base of the slab and bedrock of the dome. Once this process begins, positive reinforcement of weathering results from a reduction in surface evaporation caused by solar shading due to the overhang. Some weathered slabs take on the form of pedestal rocks, or "rock mushrooms." An illustration is shown (with explanation) on pages 40-41 in Petersen (1988).

Weathering Subaerial or Subterranean? The basal limit of decayed granite, called the "weathering front," is typically pitted and sharp on granitic rocks. The abrupt contact between relatively fresh and weathered granite is attributable to the extremely low permeability of solid granite. Weathered granite holds moisture and grus-covered areas might be expected to still be "down-weathering." But, at ER, bare areas seem little weathered, and areas where the weathering front can be examined show only minor etching. The weathering front is likely a relict of a climatic regime different from the conditions of today, as wide areas of solid, unweathered granite are exposed nearby.

No remnant of Cretaceous carbonates remains on the batholith today, but extensive remnants of Paleozoic clastic sediments exist, both as hills formed by topographically inverted grabens, and also as cap rocks (at House Mountain). Flared slopes are not common in the Llano Uplift and the absence of flares has been interpreted to mean gradual rather than episodic stripping (Twidale 1982: 146). Although the Llano Uplift forms a topographic basin, and this has been interpreted as evidence that the granite is weaker than the overlying carbonates, field evidence suggests that the reverse is true. The Cretaceous (and Paleozoic) cover of sediments has been stripped off a nonconformity, exposing the etch plain. The topographic surface of the exhumed unconformity in granite areas suggests little erosional modification in the ER area, perhaps more on the metamorphic rocks. The stripping process must haye occurred under a climatic regime in which granite would be resistant, and carbonates would be highly susceptible to weathering and erosion. Cool, wet climates would be the best possibility, perhaps Pleistocene. Wet conditions in the subsurface would continue to develop the weathering front, but residual hills and bare granite platforms would be resistant. The erosive effectiveness of runoff in a cool, min-evaporal, humid environment would readily strip the carbonate overburden, through solution and fluvial erosion.

Edwards Plateau

Figure 5. Diagrammetric cross-section of the Enchanted Rock area (Petersen 1982).

The development of Enchanted Rock may be summed up by the following statements:

1. Residual hills developed on the Enchanted Rock batholith are located along and related to the margins of the lithologically-zoned batholith. Similar lithologic-

"

8

topographic relationships may have also existed on the Early Paleozoic and pre­Cretaceous surfaces.

2. Marginal fractures have contributed to the form and positive relief of the bornhardts in the park.

3. The presence of a sharp weathering front and in situ weathered granite on Freshman Mountain and Little Rock (along with features on the west side of Enchanted Rock), suggest a mantle of weathered granite, and fit a two-stage development for Enchanted Rock (perhaps multi-cyclic). Enchanted Rock stands above an etch-plain, parts of which are exhumed.

4. The top of Enchanted Rock and the top of House Mountain, on opposite sides of the batholith, are approximately accordant, and at an elevation higher than the general level of the basal Cretaceous, suggesting positive relief in some form on the Enchanted Rock batholith prior to the Cretaceous transgression. House Mountain is capped by the Hickory Sandstone (basal) member of the Cambrian Riley Formation.

Castle Tors (Castle Kopjes): Looking from the top of Enchanted Rock over the . etched surface of the batholith, several residual hills project above the surface. Most of these hills are not bornhardts, but have a fractured character. Block or boulder-strewn, hill-sized, outcrops of massive rock are called nubbins or knolls (Twidale 1982). Angular and castellated forms are known as castle tors or castle koppies (kopjes; koppie is Africaans for a head, and colloquially is used to mean a hill). Granite hills often share the characteristics of several variations, and this is true in the Llano Uplift region, particularly in the vicinity of the Enchanted Rock Batholith.

Tent Blisters, A-tents, or Pop-ups: At a number of localities, slabs of granite that are about 15-22 em thick have apparently been laterally compressed during release. The vertical joints separating these fragments form roughly regular polygonal patterns. Some of the slabs have slightly raised edges and corners as evidence of compression. In a few places, adjacent sheet fragments have been raised up along a common side to form a tent-like structure, with the peak formed along the common joint. The height of the popped-up slabs at the tent peak is about 20 to 50 cm. Blank (1951 a) referred to the ones on Enchanted Rock as tent blisters, in the first geomorphological study of these features. They have also been referred to as pop-ups, and were called A-tents by Jennings and Twidale (1971) in a study of granitic forms in Australia. Folk and Patton (1982) attributed these forms to laterally-buttressed expansion that can only be expressed by arching. The arch fails in the process, cracking to form the central fracture at the tent peak. According to Folk's (with Patton 1982) application of Pythagorean Theorem, the sort of arching observed requires only a 1 % expansion. Further, he gives the following example, based on a one-percent expansion.

or,

50.5 m 50.5 m

100 m

Figure 6. The geometry of buttressed expansion, after Folk and Patton (1982).

9

Weathering Processes: The surface of Enchanted Rock displays a wealth of diverse weathering forms. Some of these features are classic examples of microrelief landforms commonly observed on granitic rocks in many areas of the world, others are more unusual. Granite weathering features in central Texas have been studied by several workers (Blank 1951 a, 1951 b; Kastning 1979; Folk and Patton 1982; Petersen 1982, 1984, 1988b), and the region has received some attention by C. Rowland Twidale (1982), an Australian geomorphologist known for his extensive research on granite landforms. No discussion of granite weathering forms would be complete without reference to Twidale, who has done work on granitic topography worldwide, and has produced an exhaustive survey and literature review in his 1982 book, Granite Landforms.

Granitic rocks in an arid to semiarid climate tend to be resistant (Barton 1916), but they are often deeply weathered in areas of humid, particularly tropical humid, climate zones. Twidale (1982: 24) has stated that granite weathering, "". is almost totally due to moisture attack." The process of chemical weathering has been important in the development of granite landforms in central Texas. Decomposition of minerals here follows Goldich's (1938) weathering series, and the majority of minerals in the rock are on the resistant end of the scale, the major exception being biotite. Solution also plays an important role (Begle 1978). The weathering of dikes on Enchanted Rock indicates textural variation, aplitic dikes are usually resistant and form small ridges, and pegmatitic dikes tend to be non-resistant, forming small troughs. The relationship between dikes and weathering is shown in Petersen (1988), pages 18-19.

Granite Weathering and the Formation of Grus: The traditional view of granite weathering involves initiation of decomposition by hydration of biotites and other chemical weathering of non-resistant minerals. Granular disintegration, a grain-by-grain process of disaggregation, is generally considered to be due to bloating and decomposition of weak mineral grains. An alternate interpretation of granite weathering processes has been proposed, based on study in the Liano Uplift. According to Folk and Patton (1982), weathering of granite in central Texas is initiated by physical break-up due to micro-fracturing related to buttressed expansion of the granite. They found that micro-fractures cut unweathered mica books and crystalline boundaries, providing avenues for infiltration of moisture. The orientations of microfractures were found to parallel rock surfaces, and were not related to the orientation of biotite books and cleavage directions (Begle 1978). Most rock weathering involves the combined effects of mechanical-chemical processes and this is true at Enchanted Rock. Weathering is a positive feedback mechanism, with reinforcement caused by a cycle of wetting, solution, crystal dislocation, and further penetration of water (Twidale 1982: 116). Biotite hydration remains important to rock weathering here, but it follows, rather than precedes microfracturing, the first stage of grusification (Folk and Patton 1982).

Weathering Depressions: A variety of weathering pits are found on the surface of Enchanted Rock. Surface depressions develop wherever water can pond due to level slope, disaggregation of granite, and sheeting. Depressions on Enchanted Rock vary in size from small surface pitting of a few centimeters to weathering pits having a diameter greater than 20 meters. The largest pits are located near the summit. Most weathering depressions are shallow, having a low depth-to-width ratio, and properly, are weathering pans. Structural features that pond or retard the down-slope movements of water on steeper slopes will facilitate pit development. Examples of such obstructions include raised edges of resistant aplite dikes, resistant or non-resistant xenoliths, upslope edges of exfoliation slabs, and the intersection of joints. Joints that trend up and

1 0

down the slope frequently develop into a string of small pits, generally less than 25 cm in diameter. Weathering pits associated with these obstacles are irregularly shaped and do not have the flat bottoms that characterize the pans near the summit. Factors controlling the development of either pan or pit forms were illustrated by Twidale and Corbin (1963), as illustrated in Figure 7.

(J A PIT 10 MILES NORTH EAST OF

PllOAPPA HILL.

PIT

PAN

/ .~ ~' " E

Figure 7. Pit versus pan forms, from Twidale and Corbin (1963).

Following a rainstorm the pits may overflow, forming chains of small ponds connected by the rills of a drainage system beginning to develop on some areas of Enchanted Rock. On gentle slopes close to the top of the dome, weathering pits are smaller and less numerous. Runoff in this zone generally flows unimpeded as sheetwash, or in rills. Rills are floored with a knobby crystalline microrelief of interlocking quartz crystals, because frequent wetting has dissolved, weathered, and eroded the other minerals.

Initiation of Weathering Pans: Weathering pans near the top of the dome have been greatly influenced by exfoliation. Sheeting splits the bedrock along fractures concentric to the dome surface, and many sheets are thin «2.5 cm thick). Micro-sheeting is particularly active on the summit of Enchanted Rock, and in places, thin sheets have popped up like blisters, but are unbroken. These blisters can be found by tapping on the rock, which produces a hollow sound. Initiated by the micro-sheeting process, small incipient pans have flat bottoms and vertical sides, usually less than 2.5 cm deep, reflecting the thin sheeting produced by exfoliation. Sheets apparently arch upward due to laterally-buttressed expansion (Folk and Patton 1982), disintegrate, and chemical weathering causes mineral crystals to disaggregate, forming grus. The sides of a depression left by the disintegration and removal of a micro-sheet reveal minute cracks paralleling the surface and extending into the pan walls. These bounding fractures between micro-sheets provide pathways for seepage of moisture into the walls, fostering lateral (rather than downward) development (Petersen 1988b). This is a strong contributing factor to the development of pans rather than pits. Twidale and Corbin (1963) noted that horizontal laminations were primarily responsible for the development of pan forms. Their ililfStration of the impact of laminations is shown in Figure 8.

1 1

PIT

I • '1\\"'/; II" .'.\\",1/ /J".

'" t '-. I / + ",++\ , ............. 1+++

t -+" I /+ -+ A + + -;--1-.;-" -+ + •. I •

PAN

Helqht on side wall

FREQUENCY OF WATER/ BEDROCK CONTACT

Heiqht on side wall

~{~~~~~jt~Jt~ ~II .:t_.l:: ___ ==-===r=-_--====--_t.._t... _ Time

+ VIDE ... +1 t -+ -+ ... t

ENLARGEMENT"C'l i 8 -+ -+ FREQUH1CY OF WATER/ BEDROCK CONTACT

:3 '2 1 '

l- ~~"'~~ ~ '-"=--' ~---~

Figure 8. Suggested development of pits and pans, from Twidale and Corbin (1963).

Influence of Porphyritic Texture on Pan Development: Porphyritic texture at Enchanted Rock favors the development of pan forms rather than pits for two major reasons, both relating to the groundmass. The matrix strengthens the micro-sheets, and also armors the pan floor. Neither of these conditions would exist in a non-porphyritic granite. Much of the discussion of pit development in porphyritic granite is from Petersen (1988b).

At Enchanted Rock, the matrix in the porphyry produces strong laminations-- sheets that would decompose more readily if the granite did not have an interlocking, quartz-rich matrix (Petersen 1988b). The porphyritic fabric has a nearly vertical alignment of the microcline phenocrysts (generally 1.7-3.0 cm long), reflecting the orientation of convective flow structures. Thickness of the sheets is important. If the laminations are thinner than the long axis of the mineral grains, decomposition of weaker minerals will provide avenues for moisture to seep through, and the sheet will disintegrate. The cleavage planes of the biotite and feldspars may contribute to the break-up of the thin sheet. In thicker sheets the matrix acts as a seal against water penetrating the sheet. If the laminations are about the thickness of the phenocrysts or greater, the matrix bonds the sheet laterally, inhibiting weathering and break-up of the sheet (Petersen 1988b). Generally, the stronger the laminations, the greater the tendency for pan forms. Fracture surfaces, zones of fresh granite between sheets in the side walls, are exposed to wetting and accelerated weathering. Weathering is preferentially lateral rather than downward, and pan floors coincide with the surface of a micro-sheet.

In pits that have been subjected to weathering processes for a relatively long time, the resistant quartz matrix in the floor retards deepening (Petersen 1988b). Growth of the depression is forced laterally along surfaces of micro-sheet fractures, and this reinforces the development of pan forms. Matrix armoring (Petersen 1988b) is an important influence on the granite weathering forms at Enchanted Rock. Rock surfaces with a high wetting frequency tend to be free of phenocrysts, and studded with a knobby (0.5-0.7 cm) crystalline microrelief composed almost entirely of quartz. The quartz microrelief is a

12

system of ridges enclosing small depressions left by dislodged or weathered crystals. Deepening is retarded by the hard, nearly inert quartz matrix that armors the floor. Thus, the sides of a weathering pan weather more rapidly than the floor.

Weathering Pan Development: At the beginning of pan development, the floor consists of freshly exposed, unweathered bedrock, and all of the four dominant minerals in the granite are present. As wetting and weathering progress, minerals decompose following Goldich's (1938) weathering sequence, the reverse order of Bowen's (1928) reaction series. Biotite mica, the first mineral to be weathered, becomes hydrated, followed by chalking of plagioclase and dislodging of the microcline phenocrysts. Long­term or frequent wetting dissolves the feldspars (Begle 1978).

The minerals present in a pan floor are indicators of the relative development of one pit in comparison to another (Petersen 1988b). Recent pans contain abundant feldspars, and the most recent will have fresh granitic floors with the four dominant minerals intact, including the biotite. Pans that have been subjected to long-term chemical weathering are generally floored with the resistant quartz matrix alone, as the other minerals have been removed.

The quartz floor is not, however, immune from break-up by physical weathering processes, including continued sheeting, buttressed expansion, bloating of hydrated biotite books, and frost wedging. Chemical weathering appears to operate more rapidly· than the sheeting process, as many pans are floored with a quartz matrix that is virtually free of feldspars. If the floor is broken by sheeting processes, fresh granite of the sheet or surface below is exposed, and the chemical weathering process begins again. Peeling away of the floors permits some deepening of the shallow pans.

Pitting: Frequent or intensive wetting weathers the non-resistant minerals to leave behind the quartz-rich matrix of the porphyry at Enchanted Rock. The resulting microrelief has been described as pitting by Twidale and Bourne (1976), who noted pitting as a boundary zone on granitic weathering fronts in Australia. Pitting has been uncovered in areas where artificial stripping of the weathered regolith has exposed the weathering front. Pitting often forms as differential weathering occurring beneath the regolith (Twidale 1982). Evidence at Enchanted Rock, and from my own observations at Twidale's sites in Australia, indicates that frequent or intense wetting can also cause pitting. This is particularly evident in the floors of weathering pans and rills where runoff is frequent.

Weathering Pit Ecology: The larger weathering pits on the top of Enchanted Rock exhibit many stages of plant colonization and succession. In general, the larger the weathering pit, the greater the number of plant species growing within the depression. These natural gardens (vernal pools) appear as green "islands" on the otherwise barren granitic surface of the dome. Small pits may be barren, either empty, or floored with bare grus. Larger pits may support a variety of grasses and other small plants, zoned in roughly concentric patterns which reflect changes in depth and wetness of soil. The largest pits contain prickly-pear cactus and live oak trees. The successional ecology on granitic outcrops in central Texas was discussed by Whitehorse (1933).

Enchanted Rock Cave: Several geomorphic processes have combined to produce an unusual cave along a major fracture zone. About 250 m of cave passageways have been mapped, along with 20 entrances and a lower level passage, 70 m long by 2-3 m high (Smith 1974). Kastning (1979) proposed that Enchanted Rock Cave developed as weathering processes widened major joints, and the grusified detritus was subsequently

13

removed by running water. The cave is fed by runoff from a large amphitheater-like depression upslope from the cave. The roof has formed by an accumulation of sheet fragments-- slabs that have fallen into and become wedged in the fracture. The cave is a widened joint, with passageways over, under, and around the wedged blocks. The cave is dark enough to require a flashlight or other light source to negotiate safely through the sometimes narrow openings between the blocks. Enchanted Rock Cave may be the world's longest granite cave and is certainly one of the largest of its type. Local caving groups have mapped the passageways as shown in Figure 9.

-, , , , ~ I)

Om 10 20 30 , ! ! I

Figure 9. Enchanted Rock Cave, from Twidale {1982}.

Rock Doughnuts: Some odd and as yet not well-explained weathering features were termed "rock doughnuts" by Horace Blank {1951 b}, based on a study of Enchanted Rock, and the Liano Uplift. Rock doughnuts are small circular ridges surrounding a central weathering depression. An excellent example is exposed on the west side of Enchanted Rock, just above the steep northwest-facing slope. It has the shape of a truck inner tube floating on water, but is a surface form in granitic bedrock. Blank {1951 b} suggested that rock doughnuts may be caused by case hardening of the granite surrounding a weathering pit, but admitted that he found no observable differences between the lithology of the rim and that of the central pit. Twidale and Bourne {1977}, observing these features in Australia, hypothesized that they developed originally as roughly circular bedrock platforms protected from weathering by overlying boulders. When the boulder disintegrates, a weathering pit may develop in the center of the platform, thus producing a rock doughnut.

Figure 10. Development of a rock doughnut under a residual boulder. {Twidale and Bourne 1977}

14

The bedrock beneath boulders at Enchanted Rock today displays no evidence of basal platform development. In addition, the doughnut on the top of Enchanted Rock has a vertical joint passing through the center and it is difficult to propose a corestone centered on that fracture, rather, the corestone should be bounded by fractures. Rock doughnuts remain a curious and interesting (if minor) granite weathering phenomenon.

Tafoni (Rock Lattice, Honeycomb Weathering, Alveoles): A type of surface weathering feature, called tafonl, is characterized by depressions on the sides of rock surfaces, and found at a few sites in the park. Tafoni usually occurs in massive rocks such as sandstones or granite, and generally takes the form of small «2.5 to >25 cm) honeycombs (alveolar depressions) separated by narrow ridges. The honeycomb form has not been adequately explained in the literature, but suggestions include salt fretting, differential petrology or resistance to weathering, and relationships to case hardening. At Twidale's sites on the Eyre Peninsula in South Australia, tafoni is much more prominent than it is in central Texas. Edges of sheets and slabs in South Australia often form visors and deep depressions with significant overhangs due to weathering of the undersides of sheets. At Enchanted Rock, tafoni is a minor weathering form.

Conclusion: The Central Mineral Region contains a wide variety of intriguing granite landforms-- from classic "textbook" examples, to uncommon and unusual forms. Locally, granite landforms have a questionable time of origin, as many are exhumed and relict features, reflecting the influence of mUlti-stage development. Although the Friends of the Pleistocene may appear to be anachronistic in an area of Precambrian basement with a Tertiary-Quaternary record of erosion, there is much to learn at Enchanted Rock concerning granite weathering processes and landforms, which are important to our understanding of the Quaternary.

References:

Amsbury, D. L., 1988, "The Middle Comanchean Section of Central Texas," in, O. T. Hayward, ed., Centennial F~eld Guide Volume 4, South-Central Section, Geological Society of America, Boulder, Colorado, pp. 373-376.

Barnes, V. E., 1988, "The Precambrian of Central Texas," in, O. T. Hayward, ed., Centennial Field Guide Volume 4, South-Central Section, Geological Society of America, Boulder, Colorado, pp. 361-368.

Barnes, V. E., et aI., 1972, Geology of the Llano Region and the Austin Area, Guidebook 17, Bureau of Economic Geology, University of Texas, Austin.

Barnes, V. E., Dawson, R. F., and Parkinson, G. A., 1947, Building Stones of Central Texas, Publication 4246, Bureau of Economic Geology, University of Texas, Austin.

Barton, D. C., 1916, "Notes on the Disinte\lration of Granite in Egypt," Journal of Geology, 24: 389-393.

Begle (Patton), E. A., 1978, The Weathering of Granite, Llano Region, Central Texas, unpublished Masters thesis, Department of Geology, University of Texas, Austin.

Blank, H. R., 1951 a, "Exfoliation and Weathering on Granite Domes in Central Texas," Texas Journal of Science, 3: 376-390.

Blank, H. R., 1951b, "Rock Doughnuts, a Product of Granite Weathering," American Journal of Science, 249: 822-829.

Bowen, N. L., 1928, The Evolution of the Igneous Rocks, Princeton University Press.

Flawn, P. T., 1956, Basement Rocks of Texas and Southeast New Mexico, Publication 5606, University of Texas, Austin.

Folk, R. L. and Patton (Begle), E., 1982, "Buttressed Expansion of Granite and Development of Grus in Central Texas," Zeitschrlft fOr Geomorphologle, Band 26, Heft 1: 17-32.

1 5

Goldich, S. S., 1938, "A Study in Rock Weathering," Journal of Geology, 46: 523-536.

Goldich, S. S., 1941, "Evolution of the Central Texas Granites," Journal of Geology, 49: 697-720.

Greer J. W., 1979, "An Archaeological Reconnaissance of the Enchanted Rock Area of Llano and Gillespie Counties, Central Texas," Enchanted Rock·· A Natural Area Survey, L. B. Johnson School of Public Affairs; University of Texas, Austin pp. 117-166.

Hutchinson, R. M., 1956, "Structure and Petrology of Enchanted Rock Batholith, Llano and Gillespie Counties, Texas," Geological Society of America Bulletin, 67: 763-805.

Hutchinson, R. M., 1988, "Enchanted Rock Dome, Llano and Gillespie Counties, Texas," in, O. T. Hayward, ed., Centennial Field Guide Volume 4, South·Central Section, Geological Society of America, Boulder, Colorado, pp. 369-372. .

Jennings, J. N. and Twidale, C. R., 1971, "Onginand Implication of the A-tent, a Minor Granite Landform," Australian Geographical Studies, 9: 41-53.

Kastning, E. H., 1979, "Geologic Environment of the Enchanted Rock Area," in Enchanted Rock·· A Natural Area Survey, L. B. Johnson School of Public Affairs, University of Texas, Austin pp. 19-40.

Klier, R. S., 1988, "Paleozoic Strata of the Llano Region, Central Texas," in, O. T. Hayward, ed., Centennial Field Guide Volume 4, South· Central Section, Geological Society of America, Boulder, Colorado, p; 351-360.

Linton, D. L:, 1955, "The Problem of Tors," Geographical Journal, 121: 470-487.

Linton, D. L., 1964, "The Origin of the Pennine Tors-- An Essay in Analysis," Zeitschrift fiJr Geomorphologle, 8: 5-24.

Maclay, R. and Barnes, V. E., 1976, "Field Trip Road Log Stops 1-5, October 30," in Economic Geology of South· Central Texas, South Texas Geological SOCiety, San Antonio, pp. 6-35.

Petersen, J. F., 1982, "Enchanted Rock Exfoliation Dome," in Field Trip Guide: AAG San Antonio, 1982, P. Hugill and R. Doughty, eds., Association of American Geographers, Washington D. C. pp.46-57.

Petersen, J. F., 1988a, Enchanted Rock State Natural Area: A Guide to the Landforms, Terra Cog nita Press, San Marcos, Texas (1st Ed. 1984).

Petersen, J. F., 1988b, "Influence of Porphyritic Texture on Granite Weathering Features, Enchanted Rock, Texas," Program Abstracts, International Geographical Congress, meeting in Sydney, Australia.

Smith, A R., 1974, "A Cave-- Enchanted Rock," Texas Caver, 19: 78.

Strahler, A. N., 1969, Physical Geography, John Wiley and Sons, New York p. 399.

Twidale, C. R., 1976, "On the Survival of Paleoforms," American Journal of Science, 276: 77-94.

Twidale, C. R., 1982, Granite Landforms, Elsevier, Amsterdam, 372 p.

Twidale, C: R. and Bourne, J. A, 1976, "Origin and significance of pitting on granite rocks," Zeltschrift fOr Geomorphologie, 20: 405-416.

Twidale, C. R. and Bourne, J. A., .1977, "Rock Doughnuts," Revue de Geomorphologle Dynamlque, 26: 15-28.

Twidale, C. R., Campbell, E. M., and Foale, M. R., 1987, Landforms of the Streaky Bay Area, E.L. Beck, University of Adelaide, Australia, 63 p.

Twidale, C. R. and Corbin, E. M., 1963, "Gnammas," Revue de Geomorphologle Dynamique, 14: 1-20.

Twidale, C. R. and Sved, G., 1978, "Minor Granite Landforms Associated with the Release of Compressive Stress," Australian Geographical Studies, 16: 161-174.

Whitehouse, E., 1933, "Plant Succession on Central Texas Granites," Ecology, 14: 391-402.

Zartmann, R. E., 1964, "A Geochemical Study of the Lone Grove Pluton from the Llano Uplift, Texas," Journal of Petrology, 5: 359-408.

1 6

ROAD LOG: QUATERNARY STRATIGRAPHY OF THEPEDERNALES RIVER

Day 2 of this field trip will examine the alluvial stratigraphy, sedimentology, and soils of the Pedernales River Valley between Fredericksburg and Stonewall. Rather than provide detailed stop descriptions, a complete discussion of the alluvial sequence is presented in the text that follows, and the road log below will refer to specific parts of the text as needed. A map of this area is provided following this road log, and shows the locations of all the stops. Our strategy will be to examine the Holocene sequence in the morning, have lunch at a small park on the Pedernales River near Stonewall, then examine Pleistocene deposits and solis in the afternoon.

Begin the day by driving south from Fredericksburg on U. S. 87 until we reach the last turn off on your left (east) before crossing the Pede males River. Turn onto the private farm road, and continue until crossing the Pedernales River, then follow gravel road until coming upon the large gravel quarries on your left.

STOP 1. This stop will examine a series of gravel and sand pits excavated into channel facies from late

Holocene Unit F. This stratigraphic unit represents the last major period of channel aggradation and floodplain construction by the Pedernales River, and illustrates that the modern channel and floodplain system is clearly different from that which characterized most of the late Holocene. Of special importance here are the tremendous volume of gravels present along the valley axis during this time period, which are not present along the valley axis today or in deposits of the last 1000 years. In fact, reconnaisance in the major tributaries to the Pedernales reveals that few gravels are present in the sandstone-dominated valley floor, rather they are stored further upstream near their source areas in the limestone-dominated uplands. Description of this stratigraphic unit, and measured sections from these quarries are provided on pages

16 and in Figure 12 of the text: this gravel quarry has been used to reconstruct bedforms present on the upstream end of large point bar surfaces, whereas the sand pit some 400 meters downstream was used to reconstruct bedforms on the downstream end of one of these paleopoint bars. Radiocarbon ages of 4090 ± 80 yrs BP (Tx-5534) and 840 ± 70 yrs BP (Tx-5533) have been obtained from clay lenses embedded in lateral accretion and chute channel gravels at 2.0 meters and 6.5 meters above the modern low water level in the gravel pit. Other radiocarbon ages of 4140 ± 90 (Tx-5476) and 1070 ± 50 (Tx-5475) have been obtained from the base and near the top of vertical accretion depOSits. In sum, the radiocarbon ages confirm that these deposits are late Holocene in age.

Leave the gravel and sand pits via the same private road heading east until reaching Old San Antonio Road. Turn left (north), and follow Old San Antonio Road until its junction with U. S. 290. Turn right (east), and travel approximately 2 miles on U. S. 290, then turn off the highway to your right at the Behrends Bros. fruit stand. We will continue down a small private road until reaching the Pedernales River.

STOP 2. This stop will examine: (1) the modern (post 1000 BP) depositional system of the Pedernales

River, here referred to as stratigraphic Unit G; (2) overbank facies from late Holocene Unit F; and (3) typical fine-grained facies from early Holocene Unit E. It turns out the modern river is incised and underfit, meandering about within the pattern established during deposition of Unit F - Figure 14 in the text summarizes in cartoon fashion the differences in channel geometry and depositional environments between the modern river, and that reconstructed for late Holocene Unit F. At this stop, we will first walk out on a typical modern point bar - notice it is relatively small and sand·dominated compared with that we have just seen from late Holocene Unit F. Figure 15b shows a view of this point bar from a position across the river on the late Holocene terrace. Across the river, an example of the more volumetrically dominant

overbank facies from Unit G are exposed - the top of this surface Is the modern 5 meter floodplain (see Figs. 15a and 16 in the text for more data on this exposure). Also exposed here is the erosional unconformity between this unit and older Holocene sediments, in this case Unit F overlying Unit E. Moving upstream along this cutbank, we will see typleal stratigraphic relationships between the two major Holocene fills. The upper haH of the cutbank exposes overbank vertical accretion deposits that are genetically related to the channel-related lateral accretion facies in the gravel and sand pits from the last stop. By contrast, the lower haH of the cutbank exposes what are the more dominant fine-grained sandy and muddy facies from early Holocene Unit E. Channel gravels and coarse sands were present at that time, but were not an abundant component of the overall sediment load. These exposures are illustrated in Figures 11 and 13 in the text, along with the position of radiocarbon ages.

After leaving this stop, we will travel east on U. S. 290 towards Stonewall. After about 12 miles, we will turn left on Ranch Road 1, towards the LBJ Ranch, then travel about 2 miles until stopping for lunch at some picnic grounds adjacent to the Pedernales River.

LUNCH

After leaving the lunch stop, travel back to the west on Ranch Road 1 until it meets U. S. 290, then about 200 meters to the west on U. S. 290 until reaching a small gully, which is Stop 3.

STOP 3. This stop will examine the soil developed in sandy facies of late Pleistocene Unit D. Based on the

radiocarbon age of 17,670 ± 230 yrs BP (Tx-5540) from a position just below the Ck horizon at another locality, I estimate this prOfile has had at least 15,000 years to develop. This soil, which is representative of the Pedernales Series, a udic Paleustalf (Allison et al., 1976), displays well-developed A, Bt, and Ck horizons. Both the A and B horizons have been completely leached of carbonates. Soils developed in muddier facies from this same stratigraphic unit are not as completely leached, and are typically mollisols or vertisols (the Luckenbach and Tobosa Series of Allison et aI., 1976). Refer to page 13 and Figure 10 in the text for more detail.

After leaving this stop, proceed west on U. S. 290 about 1 mile, then turn right (north) on Gellerman Lane at the Vogel Orchard fruit stand. Travel north about 1 mile until crossing the Pedernales River, then pull off on the right side of the road and park. This is Stop 4.

STOP 4. Here we will see the soil profile In sandy facies of late Pleistocene Unit C, which has been

developing for more than 30,000 yrs. Also exposed is the basal unconformity with, in this case Ordivician dolomites, and a cornplete sequence through strongly cemented Unit C channel facies. The soil is very well-developed, characterized by an A-E-Bt-K profile. Carbonates have been completely leached from the upper 1.85 meters of this prOfile, and reprecipitated below to form the petroealcic horizon. This profile is representative of what Allison et al. (1976) refer to as the Bastrop Soil (a udic Paleustaif), with the exception of the petrocalcic horizon. It Is developed deeper than the typical control section used in soil surveys, so was never described (see page 12 and Fig. 9 in the text for discussion and data on this exposure). The channel deposits exposed In the gully displays some of the same facies described from the late Holocene Unit F depositional system, including sandy sedimentary structures that are truncated upwards by a large, lenticular-shaped, gravelly chute channel fill. Across the river along the low water mark, is an exposure of Unit D channel facies - notice the high-angle tabular cross-stratified gravels dipping in the downstream direction. These are the basal facies for the Unit D terrace we visited in Stop 3, which you can see in the background. The higher surface to the north and east of this locality is what I refer to as early to middle Pleistocene Unit A, now represented in the lancjscape as a series of almost completely

dissected terrace remnants.

Return to U. S. 290, and travel westward until reaching Fredericksburg. Turn left (south) on U. S. 87 and leave Fredericksburg. We will be traveling almost to our first stop of the day, but pull off of the road on the right (west) about 0.3 miles before crossing the Pedernales River.

STOP 5. This is an exposure of early to middle Pleistocene Unii A, at the section described in the

guidebook on page 9 and in Figure 8. Easily visible is the undulatory erosional unconformity with the Hensel Sands, and the extreme degree of carbonate cementation thalis diagnostic of this Unit. Notice the residual red clayey soil developed on top of a stage 4 petrocalcic horizon, which has the characteristiC tabular or platy structures. This is another example of the landscape position typical of this unit. Everywhere around us here, the high spots are underlain by Unit A, whereas the lower relief areas are underlain by the Hensel Sands or younger alluvial deposits.

Upon leaving this last stop of the day, some may wish to return to Fredericksburg and their motel rooms to get cleaned up. Simply turn around and head back in to town on U. S. 87 (north). For those wanting to go directly to the Lady Bird Johnson State Park where the beer and BBQ will be, continue south until crossing the Pedernales River, then take the first right and travel west along the Old River Road. Continue on for a couple of miles until the first low water crossing that will again take you north towards Fredericksburg on Boos Lane. After turning north, keep going on Boos Lane for several miles until reaching State Highway 16, then turn left (south). Lady Bird Johnson State Park will be on your right (to the west). Before entering the park, notice the freshly cut and therefore well exposed outcrop of Unit A on your left. You may also notice the different colored soils in the large field in the park itself to the west. The red soil Is the Pedernales Series, developed in sandy facies of late Pleistocene Unit D, whereas the

darker brown soil is the Luckenbach Series developed in floodplain facies of that same unit.

THANK YOU FOR COMING.

ACKNOWLEDGEMENTS

I would like to thank a number of the people who have made this work and field trip possible. These include my thesis comitlee, Karl W. Butzer, Stephen A. Hall, and Gary Kocurek of the University of Texas, as well as others who contributed valuable conversation on various aspects of the Pedernales River sequence, such as Thomas C. Gustavson of the University of Texas, and William R. Farrand of the University of Michigan. Steve Hall and Tom Gustavson also organized this field trip, and took care of all of the logistical details. Salvatore Valastro Jr. of the University of Texas Radiocarbon Laboratory assisted with collection of samples in the field, then processed them to provide the radiocarbon ages reported on here. Lastly, I would like to express my appreciation to the many landowners in the Pedernales Valley, who without exception, granted access to their land. Those who allowed us to visit their property on this field trip deserve special mention. These include Terry Weirich (Stop 1), Marvin Behrends (Stop 2), Oliver Ersch (Stop 3), and Henry Beckman (Stop 4).

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QUATERNARY STRATIGRAPHY OF THE PEDERNALES RIVER

by

Michael D. Blum, Department of Geography, University of Texas at Austin

INTRODUCTION

The Pedernales River flows east 176 kilometers from the high surface of the Edwards Plateau through the Central Texas Hill Country to its confluence with the Colorado River near the Balcones Escarpment, some 22 kilometers northwest of Austin. The Pedernales drainage is bounded on the north by the Llano River, also part of the Colorado system, whereas to the south the Blanco and Guadalupe Rivers flow from the Edwards Plateau, across the Balcones Escarpment, then on to the Gulf of Mexico. Preserved in the upper part of the Pedernales Valley, near the town of Fredericksburg, is a complex sequence of episodic aggradation and incision that provides a detailed record of Late Quaternary landscape evolution (Blum, 1987). What follows is a summary of the climatic, hydrologic, and geologic characteristics of the Pedernales drainage, then an overview of the Pedernales River alluvial sequence ..

MODERN CLIMATE AND HYDROLOGY

The present climate in the Pedernales valley is classified as subtropical-subhumid (Larkin and Bomar, 1983), and supports a juniper-oak-mesquite savanna (Kier et aI., 1977; Dunlap, 1983). Mean monthly temperatures in Fredericksburg range from a low of 9°C in January to a high of 28°C in August. Mean annual rainfall in Fredericksburg for the period 1951-1980 is 72.6 cm (Bomar, 1983): precipitation maxima occur in both May and September, with a primary minimum during the winter months and a secondary low during July and August. Annual temperature is fairly constant on a year-to-year basis, but precipitation values have varied a great deal through the period of historical monitoring, with almost half of the years having precipitation totals that deviate more than 25% from the long-term mean (Carr, 1967; Bomar, 1983; see Figure 2 in the introduction to this guidebook).

This seasonal and interannual variability in precipitation is reflected in the hydrologic characteristics of the Pedernales River. Runoff data have been collected by the USGS at Johnson City (drainage area 2371 km2) since 1952, and at Fredericksburg (drainage area 971 km2) since 1978. Mean annual discharge at the Johnson City station for the period of record is 5.2 m3/s, whereas at Fredericksburg the corresponding value is 1.1 m3/s: mean annual minimum discharge for both stations approaches zero, since periods of no flow are recorded occasionally during the summer dry season. By contrast, mean annual peak discharge at Johnson City is 1291 m3/s, and ranges from a low of 4.8 m3/s in 1956 to a high of 12,971 m3/s in 1952. At Fredericksburg, mean annual peak discharge for the shorter period of record is 596 m3/s, and ranges from a low of 3.9 m3/s in 1984 to a high of 1882 m3/s in 1979. According to local residents, the flood of Aug. 2, 1978 was the highest since 1907 in the Fredericksburg area, and reached a stage of 12.7 meters compared to the officially recorded high of 10.5 meters in 1979 (USGS Open File Report). The largest discharges on record at both stations have occurred during the late spring, or the late summer-early fall due to the deep southerly penetration of slow-moving frontal storms, or the inland movement of tropical storms and hurricanes.

Although discharge records are not long enough for flood magnitude-frequency calculations, a sense of the relative frequency of discharge events can be demonstrated by examining the flow duration curves presented in Figure 1. For example, mean annual discharge at either station is equalled or exceeded on average less than 1 day in five, whereas the mean annual peak discharge occurs less than 1 day in 1000. As a number of authors have noted (e. g. Baker, 1977; Caran and Baker, 1986), extreme right-skewed flow duration curves, that result from extensive periods of low flow puncuated by episodes of flashy high-magnitude discharge, are the rule rather than the exception for the Pedernales and other streams that drain the eastern margins of the Edwards Plateau.

A specific example of the flashy high-magnitude flood events characteristic of the Pedernales River is provided by data collected by the USGS for the frontal storm event of October 19, 1985. At the

Figure 1.

Figure 2.

1000..----------------,

11111"" " ---------

11111111 " 100

---------

1111 I I 10

I ----

I I I I I I -- -- ----

I I I ""11 0.1

0.01 L.. ____________ .:::....;~..J

0.1 10 30 50 70 90 99 99.9

PERCENTAGE OF TIME DISCHARGE EQUALLED OR EXCEEDED

Flow duration curves (log/log scales) for the Pedernales River at Johnson City (upper curve; based on period 1958-1985) and Fredericksburg (lower curve; based on period 1978-1985).

1500

.. E 1000 ,e ., E' co "fi ., Q 500

o~~ ____________ ~:==-4 6 12 16 20 24

TIme (hours after 12 am Oct. 19, 1965)

Storm discharge hydrograph for rainfall-runoff event of October 19.1985 (source USGS Open File Reports).

2

Figure 3. The Pedernales River at the bridge over U. S. Highway 290 near Fredericksburg during peak discharge of October 19,1985 (above) and during normal low flow (below).

3

Fredericksburg gaging station, rainfall totals exceeded 17 cm, with the majority falling in less than 4 hours: this single storm event produced 21% of the 1985 yearly total precipitation of 80 cm. Discharge at the beginning of the storm was less than 0.5 m3/s, increased slowly over a period of 5 hours to 23 m3/s, then peaked 5.5 hours later at 1350 m3/s and a stage of 9.7 meters. Some 24 hours after the beginning of the storm, and 13.5 hours after peak discharge, flow had decreased to less than 36 m3/s (Figure 2). Figure 3 shows the Pedernales River some 2 kilometers downstream from the gaging station at peak discharge during this event, as compared with the same location at low flow.

GEOLOGIC SETTING

The Edwards Plateau is an areally extensive karstic tableland constructed of Upper Cretaceous marine carbonate rocks (Stricklin et aI., 1971; Barnes, 1981). Underlying the Plateau surface, and exposed in the Pedernales Valley, are couplets of Lower Cretaceous non-marine sandstones and marine carbonates, tectonically deformed Lower Paleozoic marine sandstones and carbonates, and the structurally complex PreCambrian crystalline basement (Bames, 1981; Figure 4). Systematic downstream changes in channel and valley morphology occur as the Pedernales River traverses these different lithologies and approaches its confluence with the Colorado River, and provide a set of geomorphic criteria for differentiation of the drainage basin into upper, middle, and lower reaches (Figure 5).

In the upper Pedernales valley, where preserved alluvial sediments are widespread, important lithologic units are: (1) the Hensel Sands, a friable first cycle arkose that has allowed for the development of a broad, gently sloping valley floor and relatively low gradient (0.0013), freely meandering alluvial stream; (2) the Glen Rose Limestone, with alternating beds of backwearing limestones and downwearing marls that demarcate valley sides; and (3) the Edwards Limestone, with chert-bearing competent beds that cap uplands within the Pedernales drainage and the extensive Plateau surface to the west. Through time, these same rock units have been the principal sources of sediment for the upper Pedernales River, with the Hensel supplying siliciclastic sands and muds, and the Glen Rose and Edwards contributing coarse limestone gravels and fine calcareous muds. Moreover, chert derived from the Edwards Limestone, which crops out only in the upper reaches of the drainage network, comprises a large percentage of the gravel-sized sediment in the modern channel and older terraces;

In contrast to upstream reaches, the middle Pedernales River below Stonewall has incised through the Cretaceous section, and maintains a very steep channel gradient (0.003) due to structural control as it flows over Cambro-Ordovician limestones and dolomites, and PreCambrian igneous rocks: the combination of steep channel gradients and the resistant nature of these rock units typically leaves channel floors sediment free (Figure 6a). The lower part of the valley, tieginning at Pedernales Falls where steeply dipping Paleozoic rocks disappear into the deep subsurface (Figure 6b), is characterized by a series of canyon-confined meanders incised into Lower Cretaceous sedimentary rocks as the Pedernales has adjusted to its base level defined by the deeply entrenched Colorado River near the Balcones Escarpment. Here, the channel has a relatively low gradient (0.0017), is floored with coarse sand- to boulder-sized sediments, and is characteristic of Central Texas streams that do most of their work during infrequent, high-magnitude storm events (e. g. Baker, 1977; 1984; Patton and Baker, 1977; Caran and Baker, 1986). Alluvial sediments more than several hundred years old, other than slack-water flood deposits (Patton et aI., 1979), are rarely preserved in the middle and lower reaches of the Pedernales Valley due to these geologic controls.

PREVIOUS WORK ON FLUVIAL SYSTEMS IN CENTRAL TEXAS

Because of the large-scale erosional nature of the Edwards Plateau, Late Quaternary sediments are not commonly preserved, and the region as a whole remains poorly understood. A number of studies have addressed the geomorphic development of several river valleys, noting episodes of stream capture (Shepard, 1979; Woodruff, 1977; Woodruff and Abbott, 1979), or the relationship between channel geometry and bedrock lithology (Shepard, 1979). but chronological aspects of the terrace sequences were rarely addressed before the investigations reported on here were initiated. An exception is the work of Mears (1953), who used soil-geomorphic relationships and archaeological evidence to develop an alluvial sequence for the Sabinal River on the southern edge of the Edwards Plateau.

4

30~0'

Figure 4.

Figure 5.

',' ,,,:

700

600

200

o 20 40 '--_____ --'-_____ ---', km

Scale

L. PALEOZOIC

PRECAMBRIAN

fauh

II.''''',,, r- 30'30'

Edwards Limestone

Glen Rose Limeslone

Hense! Sands

Sed. Rocks (undit)

Sed. Rocks (undi!j

Crystalline Rocks

down side

Geologic map of the Pedernales River drainage basin and surrounding areas (adapted from Barnes, 1981).

1'T""l'-___

UPPER RIVER

Erodable Crelaceous Sands Broad, Gently Sloping Alluvial Valley Meandering, lower Gradient Siream Channels Fill9d With Sediment

KEy-----....,

Cretaceous Edwards

Cretaceous Glen Rose

Cretaceous Hensel

Cretaceous Sycamore

Paleozoic and PreCambrian (undil)

10 - o

t,lIDDlE RIVER

ReSistant Paleozoic Carbonates Mcdarately Sloping Val!ey. Channel Is Jolnt·Conlrol!ed Channel Is Sediment Starved

10 20 30 40

SCALE

lOWER. RIVER

Erodable Cretaceous Sands Highly Dissected Steep Valley

I Channel Is Canyon-Confined Vary Coarse Bedload Serumenl

50

Km

Longitudinal profile and large-scale geomorphological characteristics of the Pedernales River (adapted from Barnes, 1981).

5

Figure 6. (a) The middle Pedernales River at low water crossing on county road connecting the small town of Sandy to U. S. 290. (b) The Pedernales River at Pedernales Falls, where the river channel falls off of the steeply-dipping Pennsylvanian limestones and enters its lower canyon-confined reach.

6

The most well-known alluvial sequence for the Central Texas area is provided by Baker and Penteado-Orellana (1977) for the Lower Colorado River below the BalconesEscarpment. Based on remotely sensed data, relative-age dating techniques, and some sedimentological work, they differentiated nine channel assemblages on five terrace surfaces. Individual channel assemblages are correlated with the last interglacial (phase 7, 6R), the Wisconsin full-glacial (phase 6, 6A, and 6B). the early Holocene (phase 5). and the late Holocene (phase 4, 3, and 2). Observed morphological and sedimentary adjustments were interpreted to reflect climatically driven changes in the magnitude and frequency of rainfall-runoff events, and the types of sediment introduced into the stream channel. Unfortunately, although this work is commonly cited as a record of climatic, hydrologic, and geomorphic change for Central Texas, there were no radiocarbon ages to support the chronological framework ..

More recently, Frederick (1987) has reported on several large exposures in Colorado River alluvium opened up during construction in Downtown Austin. Vertical accretion deposits of the Sixth Street Terrace that contained the remains of extinct Pleistocene megafauna have produced radiocarbon ages from 17,000-12,000 yrs BP. Chrono- or lithostratigraphic controls on Colorado River deposits elsewhere in the Austin area remain elusive, however, since terraces are still defined on morpho­stratigraphic criteria alone. Frederick (1987) suggested, however, that terraces in the Austin area cannot be correlated with the sequence of channel assemblages identified further downstream and beyond the Balcones Escarpment by Baker and Penteado-Orellana (1977).

Further north, in Texas and OklahOma, Ferring (1986) and Hall (1986; forthcoming) present well-dated, stratigraphically controlled evidence for relatively synchronous periods of aggradation during the late Holocene. Comparable late Holocene alluviation, terminating around 1000 BP, has been recorded in the North San Gabriel River, northeast of Austin (Hall, 1986; forthcoming). Radiocarbon ages from archaeological investigations on the South San Gabriel River record early and late Holocene sediments, but stratigraphic relationships within the alluvial sequence were not investigated (Prewitt, 1981 ).

In many of the bedrock-confined streams draining the Edwards Plateau, preserved terrace sequences are absent, particularly where channels approach the Balcones Escarpment. This led a number of authors to investigate slackwater sediments deposited by large floods as an alternative paleohydrologic record. The most detailed paleoflood sequence currently available is from the lower Pecos and Devils Rivers, near their confluence with the Rio Grande (Patton and Dibble, 1982; Kochel et aI., 1982; Baker et ai, 1983; Kochel, 1988; Kochel and Baker, 1988). Based on a number of radiocarbon ages, and paleodischarge estimates using slope-area techniques, the authors suggest that the highest magnitude, but lowest frequency floods occurred between 9000 and 3000 yrs BP, and again after 2000 yrs BP, whereas lower magnitude but higher frequency floods characterized the time periods from 10,000 to 9000 yrs BP, and 3000 to 2000 yrs BP.

THE PEDERNALES RIVER ALLUVIAL SEQUENCE

Early mapping in the Pedernales valley by the University of Texas Bureau of Economic Geology (Barnes, 1952a; b; 1966) differentiated two sets of Quaternary deposits, termed high gravels (Pleistocene) and recent (Holocene) stream alluvium. Barnes described the high gravels as "heavily calichified" and believed them to be in part fluvial, but mostly colluvial in origin and derived from retreating limestone scarps. They were thought to have at one time covered a much larger part of the valley, only to have been dissected by the Pedernales and its tributaries. Recent stream alluvium was mapped as a narrow belt along the trunk stream and major tributaries, and attributed to the modern hydrological regime. The relatively flat to gently rolling landscape in between was mapped as Cretaceous Hensel Sands,

presumably on the basiS of the reddish color of the soils. The work reported here demonstrates that alluvial sedimerts in the Upper Pedernales Valley are

more extensive than previously thought. Seven informal allostratigraphic units have been identified in the field, based on a combination of geomorphic and stratigraphic relationships, the relative degree of soil development, the morphological expression of pedogenic carbonates, and the degree of non-pedogenic carbonate cementation. Individual units can be traced and correlated using soils and topographic positions from above Fredericksburg to near Stonewall, a distance of over 40 km. An example of the areal

7

distribution of different alluvial units is illustrated by Figure 7, representing part of the alluvial valley near Fredericksburg. With reference to the mapping of Barnes (1952a; b; (966), units A and B represent his "high gravels", units F and G correspond to his "recent stream alluvium", while units C, D, and E are newly recognized.

The following description will focus on characteristics that permit differentiation of allostratigraphic units. Average topographic positions are given with respect to the modern low water channel of the Pedernales River. The overall geometry of allostratigraphic units is broadly lenticular, and unless otherwise noted, younger units are inset against older fills or the Hensel Sands. Description of clastic textures and lithofacies are made with reference to Folk (1980), and Miall (1985). Older allostratigraphic units are characterized by both non-calcic and calcic soil horizons, as well as groundwater carbonate cementation of channel-related facies assemblages in the lower part of the sequence. Soil horizon nomenclature follows Gile et al. (1966), Soil Survey Staff (1975), Birkeland (1984), and Machette (1985). Series names for soils developed on different alluvial units are taken from Allison et al. (1976). Soil texture, percent calcium carbonate, and pH were measured according to procedures outlined in ASTM (1983), and Singer and Janitzky (1986).

. .... . . ................... .. .. .. .. " :::: .. :: ..... ::::::::::::: ..

" :..:;..... .. .. .. .. . . .. . ' :><> .

Figure 7.

.. .. ::.::: .. :::

MODERN (Unit G)

HOLOCENE (Units E and F)

+ •••

:::::: + ••• . . . . ... LATE PLEISTOCENE (Unit D)

.. ......

N '---'---I

o 2 '--______ ~ ____ ~, km

Scale

CRETACEOUS HENSEL SANDSTONE

CRETACEOUS GLEN ROSE LIMESTONE

Geomorphic map of the Pedemales River near Fredericksburg, showing the surface distribution of allostratigraphic units.

8

Twelve radiocarbon ages have been obtained at present, and provide some chronometric control for the last 34,000 years. Materials processed include organic residues contained within the sedimentary matrix (e. g. White and Valastro, 1984; Haas et aI., 1986), charcoal, and pedogenic carbonates (e. g. Valastro et aI., 1968). Ages are presented in uncorrected years before present using the Libby half-life of 5568 years. In older units where radiocarbon ages are unavailable, age of deposition is estimated from the degree of development of the calcic soil horizon. Machette (1985) has summarized available data on temporal aspects of calcic soil formation, which unfortunately are less than fully understood. He shows that rates of development vary significantly over large geographic areas, but generally tend to increase with increasing supply of the calcium ion and effective annual rainfall. Since the supply of calcite is not a problem in the Pede males Valley, where skeletal carbonate grains are a major component of the fluvial sediment supply, rates of formation should reflect more on annual rainfall: it seems probable that calcic soils would develop at similar rates or faster in this area as compared with less arid environments further west. For this reason, the time frames cited in Machette (1985) are considered to be conservative, and may overestimate the age of individual allostratigraphic units in the Pedernales River.

Early to Middle Plelstocene(?) Unit A Unit A is fragmentarily represented by a series of isolated topographic highs, at varying elevations,

that are preserved due to induration by secondary carbonate minerals. The basal unconformity between this unit and the Cretaceous Hensel is often covered by slope wash or vegetation, but contacts observed are irregular and at least 14 meters above the modern low water channel.

Most exposures in Unit A show up to 2 meters of indurated gravels, horizontally laminated sands, and cross-stratified gravels and sands that grade upwards into 1 .5 meters of massive gravels and sands that float in a matrix of calcium carbonate. As shown in Figure 8 the upper third of the massive horizon is almost entirely composed of finely laminated authigenic calcite, capped by 2 centimeters of highly indurated platy structures characteristic of the stage 4 petrocalcic soils of Gile et al. (1966). A strongly developed but thin « 50 cm) residual non-calcareous soil is usually found on top of unit A in various stages of erosion. Where present this soil may consist of a single grain to granular reddish brown (5YR4/4) loamy A horizon, overlying 30 cm of a red (2.5YR 4/6) clay Bt horizon with strong coarse blocky structures (Hensley Series).

The massive morphology of the primary clastic sediments in the upper half of the section is due to the displacive growth of authigenic carbonate associated with the formation of petrocalcic soil horizons (Gile et al.,1966). At the time Barnes (1952a; b; 1966) originally mapped this area little was known of the displacive nature of carbonate precipitation in the soil zone, and he attributed the massive fabric of the high gravels to deposition by colluvial agencies. With the lenticular geometry observed in outcrop, the presence of siliciclastic sands derived from the Hensel, and preserved primary sedimentary structures in the lower portion of this unit, a fluvial origin is clearly indicated.

An age estimation for the Unit A alluvium can be based on the degree of development of the calcic soil profile, since this unit is considerably older than the range of radiocarbon dating, and no fossils have been discovered. Such an estimate is necessarily somewhat tenuous, in part because of the uncertainty regarding rates of calcic soil development, but even more so because the present appearance of this unit suggests a complicated geomorphic history. A possible sequence of events might include: (1) deposition followed by valley incision and floodplain abandonment; (2) progressive soil development and cementation by carbonate minerals during subsequent periods of alluviation; (3) gradual downwasting as the unit became topographically isolated by long-term valley incision; and (4) continual translocation of carbonates down through the profile resulting in the present petrocalcic horizon. Bearing these problems in mind, petro calcic soils with this degree of development are not known to have formed in less than 105 years, with most taking considerably longer (Machette, 1985). Given such a time frame, Unit A is conservatively assigned to the Middle or perhaps Early Pleistocene.

Middle Pleislocene(?) Unit B Exposed in several high cutbanks along the Pedernales River are a series of what had been

mapped as high gravels, but occur at a consistently lower elevation than Unit A. The basal unconformity between this Unit B and the Cretaceous Hensel is commonly visible, and averages 8 meters above the modern low water channel. Maximum thickness observed to date at an exposure 400 meters downstream

9

..... o

Figure 8.

Texture (cum xl CaC03 (Xl pH o 25 50 75 100 30 50 75 100 6 7 8 9

~ e '" -.,

o

.§ 0.5 Q) u ., -.... ::I

'" :l: o a; .Q

.<:: -a. ., c

1.5

;:A.;::::::::;::;;:-:: !!jj

::.::~::: ...... :::.:,,---=--~ : B2t·· =~====t=t===t ::....... =-=-=--=-::-=-=:

no quantitative data

11K

KEY,

-..,L- _L . ...l...-L -L....J.... -L_I ..J.. ..J.. ..J.. . ..J.. ..J.. ..J.. ..J.. . . ..J.. ..J.. -L -L...J......L'

....L...,L ....L-1-'

-L...L....L -1-' ...L....L....L ....L .

....L ...J.... ...L-1-'

...L -L...l...-1-'

..J.. ..J...

~~.~.~~ ::,:.(~:'~: '" =3===E cloy « 2/1l

(a) Close·up view of petrocalcic horizon and overlying non-caicic soil from Unit A (Hensley Series). and (b) laboratory determinations of texture. % calcium carbOnate. and pH.

from the U. S. 290 bridge is 6 meters. At this locality, 0.5 meters of Indurated, imbricated cobble-sized gravels rest unconformably on the Hensel, followed by up to 5.5 meters of indurated to strongly cemented, cross-stratified sandy gravels. Channel facies from Unit B may be unconformably overlain by overbank sediments from Unit C or Unit D (see below). or have a thin, well developed non-calcareous soil, similar to that formed on Untt A (Hensley Series).

Differentiation of Unit A from Unit B does not conflict with the previous areal geologic mapping by Barnes (1952a; b; 1966), given the more specific objectives of this study. Barnes clearly recognized that some of the "high gravels" adjacent to the Pedernales River were fluvial in origin, and less "calichified", but thought they represented reworked material deposited by streams draining the extensive colluvial deposits. The rationale for subdivision is the two units represent distinctly different periods of fluvial activity, and they can be distinguished in the field on the basis of topographic position and lithologic criteria, namely the presence (Unit A) or absence (Untt B) of an extremely well developed petrocalcic soil horizon.

Estimates of the age of Unit B are difficult to make, since it too is beyond the maximum effectiveness of radiocarbon dating and has as yet yielded no faunal material. Moreover, there are no in situ calcic or non-calcic soils that can be used as a basis for estimation. Therefore, Unit B can only be bracketed between Unit A, somewhere in the early to middle Pleistocene(?), and the next younger period of deposition represented by Unit C, of probable late Pleistocene age.

Late Pleistocene Unit C Coarse-grained facies of Untt C are exposed at several cutbanks on the Pedernales, and along a

number of small tributaries. The top of the section often is partially eroded and covered by thin veneers of younger Holocene sediments, although complete composite vertical sections have been examined at several locations. Untt C rests unconformably on the Hensel Sands, with an irregular contact at 1-4 meters above the modern water level, and in most places appears to have aggraded over 10 meters of sediment. In several locations, large circular depressions containing lacustrine clays are present on terrace surfaces

of Unit C. Most exposures in Unit C show up to 7 meters of strongly cemented to indurated, interbedded,

cross-stratified sandy gravels and sands, that grade upwards into 1.5 meters of massive sandy gravels, gravelly sands, and sands dispersed in a matrix dominated by nodular and massive calcium carbonate. This massive horizon is pedogenic in origin, and meets requirements for the stage 3 petrocalcic soil of Gile et al. (1966) and Machette (1985). Above the K horizon are 2 meters of a well developed, non-calcareous solum with an A-E-Bt profile (Figure 9a). The A and E horizons comprise 50 cm of brown (10YR 5/3) sandy loam, grading into a pale brown (10YR 6/3) loamy sand, whereas the Bt horizon consists of 1.5 meters of reddish brown (5YR 4/4), sandy clay loam and yellowish red (5YR 5/6), sandy clay, with coarse blocky structures (Bastrop Series). The transition from the K horizon to the overlying non-calcareous solum is abrupt but conformable. Trends in soil texture, % calcium carbonate, and pH are summarized in Figure 9b.

There is considerable variability in the soil profiles developed on Unit C due to the original depositional texture of the alluvium. Where parent materials Included more silts and clays due to deposition in distal floodplain environments, soils have developed under less well-drained conditions, and they commonly exibit a mollic epipedon (Luckenbach Series), and leaching of carbonate minerals is less complete. A more extreme variant is also found, where the original depositional texture was dominated by expandable clays, and the resulting soil is a vertisol without distinct horizons (Tobosa Series).

An isolated clay-rich lense embedded in sandy gravels at 4.5 meters above the modern low water channel, and 1.5 meters above the base of Unit C, was processed for radiocarbon dating of the organic fraction. The sample was determined to be 33,020 ± 1620 yrs BP (Tx-5542): until other assays are available this should be regarded as a minimum age. Petrocalcic horizons with this degree of development are not known to have developed in less than 50,000 yrs. (Machette, 1985), with most taking considerably longer. Based on the single radiocarbon age, and the available information on calcic soil development, Unit C is late Pleistocene, but probably no younger than what would be middle Wisconsin in the mid-continent.

1 1

.... [\)

Figure 9.

o

0.5

" .... '" -'" .§

'" " ., -.... '" '" 3= 0 0; 1.5 .Q

.<:: Q. ., '"

2

2.5

Texture (cum %) CaC03 (:t)

o 25 50 75 100 0 35 70

:":: .~:: .. :::.::.::: .. :::~ -~:: - - -- -r'" A ·················t E~J t :::::-E :::::::::::::::::::::::

;::;:::::::::::;::::::::::., ~--

',iT ~~~t

jr:~~~~r'

_"'0 __

'" .<:: u '" '" '" ., -'" c: o .Q .... '" __ u .

(a) Photograph of non-calcareous A, E, and Bt horizons of the Bastrop Series soil developed in sandy facies of Unit C at a location 50 meters to the north of the Pedernales River, and 30 meters to the east of Goehman Lane, on property owned by Henry Beckman. Top of K horizon visible at bottom of exposure. (b) Laboratory determinations of texture, % CaC03, and pH for the Bastrop Series soil at same location.

pH

6 7 8 i

=~=:-I",

l .= - .... -'"

I! l~

_J ~ II I' I I I

Late Pleistocene Unit 0 Relatively good vertical sequences through Unit D are exposed at many locations along the

Pedernales River and in many small tributaries. Basal coarse-grained facies rest unconformably on the Hensel Sands, with a sharp but irregular contact 1-3 meters above the modern low water channel. Topographically, Unit D forms a distinct terrace with respect to younger fills at 12-13 meters above the modern river, and most commonly merges Imperceptably with the Hensel Sands away from the valley axis. Occasionally Unit D is inset against Unit C, but such contacts are rarely exposed. Similar to Untl C, terrace

surtaces may contain localized depressions filled with lacustrine clays. Most exposures in Unit D show 0.5 meters of indurated, imbricated basal gravels, overlain by 5

meters or more of strongly to weakly cemented, cross-stratified to horizontally-stratified sandy gravels and gravelly sands. This in tum is overiain by up to 2 meters of massive oxidized light reddish brown (5YR 6/4) medium to fine sands, that grade into 3.5 meters of a well developed soil wtlh an A-Bt-Ck profile (Figure 10a). The A horizon consists of 40 cm of strong brown (7.5YR 4/6), generally single grain to granular loamy sand, whereas the Bt horizon is 1.1 meters of yellowish red (5YR 4/6) sandy clay, with strong, coarse blocky structures (Pedernales Series). Both the A and Bt horizons have been leached of carbonate minerals. The Ck horizon matrix Is a light reddish brown (5YR 6/4) calcareous loamy sand, with medium columnar to prismatic structure and common pedogenic carbonate nodules typical of the stage 2 calcic soil of Gile et al. (1966). Trends in soil texture, % calcium carbonate, and pH are summarized h Figure 10b.

Observed variations in soil development due to the original depositional texture occur in both the coarse and fine directions. Vertical sequences dominated by up to 10 meters of gravels, sandy gravels, and coarse sands are relatively common, as are those where the upper 5 meters are dominated by silts and clays. The most significant differences are with respect to the depth of leaching of carbonates, and the degree of development of the Ck horizon. The coarser variant, due to more rapid leaching, will tend to have a better developed Ck, wtlh nodules and soft masses of carbonate filling many interclast areas, and it may be deeper in the profile (also mapped as the Pedernales Series). In contrast, due to poor drainage conditions the upper part of the solum in soils developed in silts and clays have not been completely leached of carbonates, and may have an A-Bt-Btk-Ck profile (Luckenbach Series). An extreme variant dominated by expandable clays is characterized by a vertisol that lacks distinct horizons (Tobosa series).

Two radiocarbon ages have been obtained from Unit D, and provide an initial, Internally consistent temporal framework. Processing of organic compounds bound to the clay fraction of loamy sediments in the lowermost part of the Ck horizon has yielded an age of 17,670 ± 230'yrs BP (Tx-5540). Carbonate nodules from this same sample yielded an age of 5200 ± 340 yrs BP (Tx-5543; corrected for a13C), which represents a minimum age for depostlion of Untl D, since it implies that the calcic horizon had at least partly developed by that time. Stratigraphic constraints place deposttion of Unit D between middle Wisconsinan Unit C, and the latest Wisconsinan and early Holocene Unit E (see below). In other parts of the Western Untted States calcic soils with similar Stage 2 morphology have developed in sandy-textured sediments deposited during the late Pleistocene (Machette, 1985). In sum, based on consideration of radiocarbon ages, stratigraphic constraints, and known rates of development of calcic soils from other areas, Untl D is late Pleistocene in age, perhaps contemporaneous with full-glacial conditions in the mid-continent.

Latest Pleistocene and Early Holocene Unit E Examination of cut banks along the Pedernales River demonstrates the presence of a

depositional episode that is unexposed at the surtace, having been erosionally truncated then buried by overbank sediments from late Holocene Unit F. Subsequent investigations revealed that this stratigraphic relationship is consistent and applies to the volumetrically dominant fine-grained facies, as well as the subordinate coarse-grained facies of what is here referred to as Unit E. Regardless of texture, the basal unconformity with the Hensel Sands is irregular and commonly visible at low water.

Coarse-grained facies generally consist of 1 meter of weakly cemented, imbricated gravels overlain by up to 4 meters of oxidized, weakly cemented to unconsolidated, horizontally-stratified to cross-stratified sandy gravels and gravelly sands, upon which rest unconformably 4 meters of overbank muddy sands from Unit F. The volumetrically dominant finer-grained facies, illustrated in Figure 11, demonstrate the lateral variation seen in outcrop. The section to the right of the small arroyo fill consists of

13

Figure 10.

o

0.5 ~

E ., ~ ., -5 ., u ., t: => '" l= 0 Q;

1.5 .c

'" ~ Q. ., 0

2

2.5

'" '" .c: u ., ., '" '" ~ " --5--_· -e " u

pH

676

-- t I I I I I

f

(a) Photograph of the soil developed in sandy facies of Unit D (Pedernales Series) at a gully exposure 100 meters south of the Pedernales River near the junction of Ranch Road 1 with U. S. 290 just west of Stonewall. (b) Laboratory determinations of texture, % CaC03, and pH from a gully exposure on Edward Stehling's property 0.5 km. to the north of junction between U. S. Hwy. 290 and the Cain City Rd (also the Pedernales Series).

14

5.2 meters of pale brown (10YR 6/4) muddy sands and sandy muds with a partially preserved, weakly developed buried soil overlain unconformably by 3 meters of brown (10YR 5/3) muddy sands from late Holocene Unit F. By contrast, the section on the left contains 0.5 meters of horizontally stratified gravels, overlain by 5 meters of light brown (7.5YR 6/4) oxidized muddy sands that were erosionally truncated lower in the section, hence the buried soil is not preserved.

Radiocarbon ages of 10,550 ± 130 BP (Tx-5537) and 7150 ± 90 yrs BP (Tx-5538) have been obtained at 20 cm and 3.6 m above the modern low water level (see Figure 11), suggesting that most of Unit E deposition took place during the latest Pleistocene and early Holocene. Several teeth and a small piece of a skull belonging to an extinct Pleistocene horse (Equus sp.) were found at 3.5 meters above the modern water level to the left of the small arroyo fill in Figure 11, at the same level that produced the radiocarbon age of 7150 ± 90 yrs BP. As Meltzer and Mead (1985) report, the last reliable radiocarbon ages for Equus are older than 10,000 yrs BP, which suggests the teeth may have been locally reworked and incorporated into younger deposits.

Figure 11. Photograph of common fine-grained facies from Unit E, as exposed in cut bank 300 meters upstream from bridge on U. S. 290 that crosses the Pedernales River. Unit E is overlain unconformably by overbank facies of late Holocene Unit F.

15

Late Holocene Unit F The last major period of widespread channel aggradation and floodplain construction in the Upper

Pedernales Valley is represented by Unit F, and constitutes the majority of what had been mapped by Barnes (1952a; b; 1966) as "recent strearn alluvium". Topographically, Unit F is represented by the undissected +9 meter terrace, against which sediments resulting from the modern hydrologic regime are inset (Unit G below). According to local gravel-pit operators, the unconformity with the Hensel Sands can be up to 2 meters below the water line, suggesting aggradation of more than 10 meters of gravels and sands.

Early field reconnaissance demonstrated the modern stream is deeply incised due to geologically recent events, and that the large-scale plan view geometry of the modern river is a relict feature related to the period of channel aggradation and floodplain construction represented by Unit F. According to Dury (1965), this would place the modern Pedernales River among the ranks of underfit streams. Preserved point bar surfaces and channel margin levees, in combination with extensive exposures due to quarrying operations, provided an opportunity to reconstruct sedimentation styles in some detail (Blum and Valastro, in press). Measured sections from the upstream gravel-dominated, and downstream sand- and gravel-dominated parts of one of the large paleopoint bar are presented in Figure 12, while overbank facies are illustrated in Figure 13. To briefly summarize the results of this reconstruction, the Pedernales channel during deposition of Unit F was perhaps three times as wide as it is today, as a result of adjustmenf to a gravel-rich sediment load, and sedimentary processes were focussed on point bars some 5 times the size of those that characterize the modern stream. Lower bar surfaces were constructed by lateral accretion, and covered by a variety of gravelly bedforms on the upstream end, which interfingered with sandy bedforms downstream, whereas upper bar surfaces were transected by large chute channels that filled with prograding gravelly chute bars. Vertical accretion of fine sand and mud occurred on stable vegetated surfaces in channel margin levee and flood plain settings on the outside of meander bends, or in far downstream and basinward settings adjacent to point bar surfaces, but they were subordinate to the ubiquitous coarse gravel and sand deposited on large lateral accretion surfaces and in large chute channels. A schematic model of the late Holocene depositional system is presented in Figure 14b.

In terms of field identification this unit can be clearly recognized, since pedogenic processes have only marginally altered original depositional characteristics. Weakly developed soils with A-Cu or A-Bw-Cu profiles, slight leaching of skeletal carbonate minerals in the A or B horizons, very little color change with the exception of organic matter accumulation, and no oxidation in the C horizon characterize both the coarse and fine-grained facies (see Figure 13; the Guadalupe Series).

At present, radiocarbon ages of 4140 ± 90 yrs BP (Tx-5476) and 1070 ± 50 yrs BP (Tx-5475) have been obtained from vertical accretion deposits 'of Unit F. The first sample consisted of culturally-derived charcoal 5.5 meters above the base of the section illustrated in Figure 17 (0.5 meters above the diffuse unconformity with Unit E). The second sample consists of charcoal taken from an extensive natural fire found in muddy sands deposited at the downstream end of a meander bend 8.2 meters above the modern water level, and 0.5 meters below the terrace surface. Additional ages of 4090 ± 80 yrs BP (Tx-5534) and 840 ± 70 yrs BP (Tx-5533) have been obtained from clay lenses embedded in lateral accretion and chute channel gravels at 2.0 meters and 6.5 meters above the modern low water level in the Weirich gravel pit. In sum, radiocarbon ages do not completely bracket Unit F, but they demonstrate depOSition took place in the second half of the Holocene, mostly between ca. 4500 and 1000 yrs BP.

The Modern Stream Regime· Unit G Unit G consists of the narrow, inset band of sediments adjacent to the modern Pedernales that

accumulated during and after the downcutting phase that led to abandonment of the Unit F channel and floodplain. This unit is dominated by massive, ripple-laminated, or horizontally-stratified brown (1 OYR 5/3) muddy sands deposited by vertical accretion, and inset against sediments from Units E and/or F. The geomorphic surface associated with Unit G vertical accretion deposits is the active +5 meter floodplain. Although there has been little gravel delivered to the Pedernales valley axis from the tributary network since abandonment of the Unit F channel and floodplain, there are limited channel reaches where lateral accretion of small gravelly and/or sandy point bars has occurred, but they are some 5 times smaller than what characterized the Pedernales River during deposition of Unit F. Figure 14a presents a schematic model for depositional environments associated with the modern Incised and underfit channel, as

16

A.

--' ---J

Figure 12.

\i / e>

j\ / ~ Plan View / ~ \ Weirich Point Bar

"­

'" l: J:

\ /

~!l { 0.5 ,\

I II I

B.

Measured sections from the Weirich gravel and sand pits. The small letters stand for facies codes, with Gh = horizontally stratified gravels, Gph = high·angle tabular cross-stratified gravels, Gpl = low-angle tabular cross-stratified gravels, Gt = trough cross-stratified gravels, Sh = sandy planar beds, St = trough cross-stratified sands, Sr = ripple·laminated sands, and Sm = massive sands and/or muds.

8

6

4

2

o

3 CD CD en

'" a-D < CD

3 D "­CD :; n ::r § :J ~

-'

CD

Figure 13_

Texture (cum %) C8C03 (%)

o 25 50 75 100 0 20 40 o l ........... J F.:J , -' ~ ~ :: .. :: . .:: .. :: .. :: .. :: ........ 1,::=:::::::::::: I.:::::::::::::'

0.5

0; '-., -., .5 .,

••••••••••••••• <> 0 -'-'" ., 3:

t 0 Q; 1.5 ..0 .<: -Co

'" 0 ::t::::::::\::::!

2

:,

Overbank facies from Unit F, here lying unconformably on early Holocene Unit E, as exposed in cut bank 150 meters upstream from bridge on U. S. 290 that crosses the Pede males River. Laboratory determinations of texture, % CaC03, and pH for weakly developed soils and sandy to loamy civerbank facies of Unit F as exposed in cut bank 200 meters upstream from bridge on U. S. 290 that crosses the Pedernales River.

'-L -L...L.'

'-1--L-1..'

'-1--L --t.: '-L -L...J...:'

'-L...L -1--'

'--1---L...L: , • ...L -L...L ...L~ :...L.....L.-L...L

. ...L ...L...L'

~-L -L.-L'

• ...L. -L-L "-1--L -L-1-,

'-1--L -L --L: "-1--1--L -1-_,

'-1-...L -L....L .

"-L ...L-L -1-,

'-1--L-L-1-,

:..L -L -L -L.

:...L -L-L -1-,

I • ..L --L,.L -L..Ji

1

- -'- -'­-'- -'- . "-1--L -L ....L.

"-L...L..L -L,

-'- -'-

pH

6 7 8 9 ,

gravels [?-<i 9'g diffuse gravel sheets, , ":::;:o-",'}.;;l transverse bars. long!tudmal bars

satldy gravels ~ transverse bars. longitudinal bars ~ chute channel fills

gravelly sandS f;·:·.\::::=j chute channel fills, ,,-,:;q;;::;':-:-\ dunes, sand waves

sands W~~t??}l g~a~~Sb~~~d waves,

low water channel

low waler channel

muddy sands, ~ .......... -. ripples.la1l)inated ..

sanoy mUdS L.2J massive. b,oturbat!oin~il~~~~~II~~i~~i muds !==:=:==l,a,minated, massive _:_:_:_:_ bloturbal!On

A.

8.

Figure 14. Schematic models of the Pedernales River during deposition of Unit F. from ca. 4500 to 1000 yrs BP; as compared with the modern incised and under/it stream (Unit G).

19

compared with the Pedernales River during deposition of Unit F. Figure 15a illustrates vertical accretion deposits and the modern +5 meter floodplain, as well as common stratigraphic relationships between Units E, F, and G, whereas Figure 15b shows a modern active point bar of the Pedernales River. Figure 16 documents the texture, % calcium carbonate, and pH typical of a representative section through overbank facies of Unit G.

A radiocarbon age of 830 ± 70 yrs BP (Tx-5532) was obtained from basal, organic-rich muds that unconformably overlie channel gravels from Unit F at 1.1 meters above the modern water level. Additional ages of 420 ± 80 yrs BP (Tx-5465), 190 ± 90 yrs BP (Tx-5467), 150 ± 50 yrs BP (Tx-5466), and 130 ± 190 yrs. BP (Tx-5468; very small sample) were obtained from carbonized wood directly across the river at positions shown in Figure 15a. The statistical error associated with the oldest radiocarbon age from Unit G (810 ± 70 yrs BP; Tx-5532) overlaps with that of the youngest age obtained from Unit F (840 ± 70 yrs BP; Tx-5533), and may signify that point bar surfaces of Unit F were still active as channel incision began. Others are internally consistent, and confirm that Unit G is a product of the past 5-8 centuries of activity on the Pedernales River. Radiocarbon ages from both the upper part of Unit F, and the basal sediments from Unit G suggest that the period of channel incision and floodplain abandonment was very short, probably less than 200-300 years.

SUMMARY OF THE PEDERNALES RIVER STRATIGRAPHIC FRAMEWORK AND SEDIMENTOLOGICAL INTERPRETATIONS

The Pedernales River contains a detailed and partially dated record of changing stream behavior through the Quaternary period, a record previously undocumented for Central Texas in general, and the Edwards Plateau in particular. A schematic valley cross-section illustrating known stratigraphic relationships between the different alluvial units is presented in Figure 17. To summarize, Unit A occurs at varying topographic positions, with surtaces ranging in elevation from 18 to well over 25 meters above the modern stream, and is unrelated to the mcdern integrated drainage pattern. Similarly, the elevation of Unit B varies, with observed basal unconformities ranging from 6 to 10 meters above the modern stream. However, unlike Unit A this period of net depositional activity took place within the modern drainage pattern. Given the highly variable topographic elevations, it may be that Units A and B each represent multiple episodes of valley aggradation and incision, but with available outcrops they cannot be further differentiated. By contrast, younger Units C, D, E, F, and G represent individual periods of fluvial activity that are bounded by erosional unconformities, and that took place within the present integrated drainage network.

From a sedimentological perspective, middle Pleistocene or older Units A and B are difficult to analyze, since their topographic position has made them prone to dissection and downwasting as the Pedernales River has cut deeper into its valley through time: coarse-grained gravelly and sandy facies that cement faster are selectively preserved, and the present distribution of facies may not be representative of the fluvial system at the time of deposition. However, measured and observed sections in late Pleistocene Units C and D suggest a style of sedimentation analogous to the depositional model developed from late Holocene Unit F (see Figure 14). Coarse gravelly and sandy lateral accretion deposits with inset chute channel fills constitute the most widespread facies assemblage, although overbank vertical accretion deposits are present as well. In sum, extended episodes of coarse-grained channel aggradation and floodplain construction represented by late Pleistocene Units C and D, and late Holocene Unit F record time periods when the Pedernales River was supplied with an overabundance of coarse gravels delivered to the trunk stream from their limestone-dominated source terrains in the upper reaches of the tributary network.

In contrast to gravel-dominated depositional units, vertical accretion of muddy sands and sandy muds produced the dominant lithofacies assemblage represented in latest Pleisocene to early Holocene Unit E. Coarse-grained channel-related sandy and gravelly facies are present, and attain thicknesses of up to 4 meters, but are a less significant component of the overall facies assemblage. Unit E represents a time periOd when gravelly sediments derived from the upper reaches of the tributary network comprised a smaller percentage of an otherwise abundant sediment supply, and the resulting sedimentation style consisted of localized lateral migration and accumulation of bedload, but mostly slow vertical accretion of overbank sediments in stable channel margin and floodplain settings. At present, other periods of net

20

Figure 15.

I' '

OJ ,'.

(a) Photograph of measured section through Unit G, at an exposure 80 meters upstream from bridge that crosses the Pedernales River on U. S. 290. Location of radiocarbon ages are as shown. (b) Modem point bar of the Pedernales River at same location, but 50 meters further upstream.

21

Figure 16.

~ 18 <1> ro S 15

" 0 12 -'

E <1> 9 TI 0 :;; <1>

6 > 0 3 .0 « ~ 0 2 <1> -3 :;;

Figure 17.

Texture (cum %) CaC03 (%) pH

0 0 25 50 75 1 00 0 20 40 6 7 8 9 .. " . .. .. ~ .. .. .. .. .. .. .. .. .. .. .. .. .. .. -L .. .. .. .. .. .. .. -

'" .. .. .. .. .. .. .. .. .. .. .. .. .. .. - -L I- .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. =:: -L-L Q) .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ~ .. .. .. .. .. .. .. .. .. .. .. .. .. ..

-L-L -Q) .. .. .. .. .. .. .. .. .. .. .. .. .. .. E .. .. .. .. .. .. .. .. .. .. .. .. .. -L ~ .. .. .. .. .. .. .. .. .. .. .. .. .. .. -.. .. .. .. .. .. .. .. .. .. .. .. .. .. -L Q) 0.5 .. .. .. .. .. .. .. -L-L (j .. .. .. .. .. .. .. .. .. .. .. .. .. .. -., .. .. .. .. .. .. .. .. .. .. .. .. .. .. -L-L - .. .. .. .. .. .. .. l- . -.. .. .. .. .. .. .. .. .. .. .. .. .. " .. .. .. .. .. .. .. .. .. .. .. .. .. .. -L-L '" .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ;l: .. .. .. .. .. .. .. .. .. .. .. .. .. .. - -L -L ..1 0 .. .. .. .. .. .. .. .. .. .. .. .. .. .. a; .. .. .. .. .. .. .. .. .. .. .. .. .. .. - -L -L ..1 .c

.. .. .. .. .. .. .. .. .. .. .. .. .. ..

i~ .. .. .. .. .. .. .. -L -L ..1 .c .. .. .. .. .. .. .. .. .. .. .. .. .. ..

~ .. .. .. .. .. .. .. .. .. .. .. .. .. .. Q. .. .. .. .. .. .. .. .. .. .. .. .. .. .. -L-L Q) .. .. .. .. .. .. .. .. .. .. .. .. ..

0 .. .. .. .. .. .. .. .. .. .. .. .. .. -L-L .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. -L-L .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. -L ..1 1 .5

.. .. .. .. .. .. .. -L

Laboratory determinations of sediment texture, % CaC03, and pH at this same locality. These sediments represent what deposits of older fine-grained facies were like, in terms of color, calcium carbonate content, and pH before pedogenic processes acted upon them.

Early to Middle Pleistocene Middle Pleistocene

Late Pleistocene

Early Holocene

Late Pleistclcerle

Schematic cross-section of the Pedernales River valley, summarizing common geomorphic and stratigraphic relationships, and the relative position of radiocarbon dates.

22

aggradation dominated by similar fine-grained vertical accretion deposits have not been identified. Compared with major net aggradational episodes, Unit G represents the deposits of a laterally

confined, incised, and under/it stream that is a result of the downcutting episode of the last millenium. Fine-grained sediments deposited by vertical accretion in the overbank environment constitute the most important facies assemblage, whereas thin channel-related facies consisting of sandy and gravelly bedforms are present, but relatively minor constituents of the overall depositional system. Areas of deposition during previous periods of down cutting probably were also limited in their lateral and vertical extent, and were destroyed during subsequent periods of lateral channel migration and and aggradation, and floodplain construction. The episodes themselves are recorded by erosional unconformities that bound individual aggradational units.

Regional Correlations The tentative chronological framework developed above suggests that periods of net channel

aggradation and/or floodplain construction occurred throughout the early and middle Pleistocene (Units A and B), during the late Pleistocene (Units C and 0), the latest Pleistocene and early Holocene (Unit E). and the late Holocene (Unit F). Valley incision has been characteristic of the last millenium, and is represented stratigraphically by Unit G. Because of the small number of radiocarbon ages, correlations of much of this sequence with other alluvial chronologies are premature at this time. However, the tentative chronostratigraphy outlined above is similar to that presented by Ferring (1986). and Hall (1986), for parts of Texas and Oklahoma. This similarity is most apparent for the late Holocene aggradational episode present in the North Texas and Oklahoma examples, the North San Gabriel River of Central Texas, and on the Pedernales River. In each case, this period of aggradation was followed by channel inCision that began around or shortly after 1000 yrs BP (Hall, forthcoming).

Recent geoarchaeological work on the Upper Colorado and Concho Rivers, some 200 kilometers north and west of the Pedernales drainage, has produced an alluvial sequence that is very similar to that described above (Blum, 1989). The main components of this alluvial sequence are two dissected terrace remnants of probable early to middle Pleistocene age, two partially dissected late Pleistocene terraces, and a complex Holocene valley fill. The younger portion of this record Is temporally constrained by more than 50 radiocarbon ages, and includes an extensive late Pleistocene terrace with soil profiles characterized by strong argillic and calcic horizons, an early Holocene fill with a moderatly well-developed soil profile characterized by incipient argillic and calcic horizons, and a late Holocene fill characterized by weakly developed soils with A-Cu profiles and no secondary carbonate morphology. As in the Pedernales valley, late Holocene sediments aggraded to a position topographically high enough to bury the early Holocence fill. Also like the Pedernales valley, the modern under/it channel and associated depositional environments are inset Into the late Holocene terrace sur/ace, and represent the last millenium of activity.

Though they had no radiocarbon dates, Baker and Penteado-Orellana (1977) postulated a similar sequence of events for the Colorado River below Austin. However, recent reexamination of the Lower Colorado alluvial sequence, and comparison with radiocarbon-controlled deposits from the Upper Colorado, Concho, and Pedernales Rivers, indicates that previous mapping and differentiation of terrace sur/aces and channel assemblages was inconsistent, and the ages assigned were greatly overestimated. Portions of their late Pleistocene channel assemblages 6, 6a, and 6b are clearly late Pleistocene in age

(A-Bt-Ck profiles), with other portions probably early Holocene (A-BUBw-Ck profiles): but the majority of the extensive terrace sur/aces with preserved channel morphology are characterized by weakly developed soils similar to those developed in late Holocene deposits in the Upper Colorado, Concho, and Pedernales River (A-Cu or A-Bw-Cu profiles; the Bosque Series of Baker, 1979). Moreover, their entire Holocene sequence (channel assemblages 5, 4, 3, 2, and 1) is now believed to represent the depoSits of an incised and under/it channel and floodplain systern that developed during the last 1000 years or so (Blum and Valastro, in press). These conclusions are substantiated by recent temporally diagnostic archaeological materials found in Colorado River alluvial depOSits in Austin that were previously mapped as Pleistocene in age (Espey, Huston, and Associates, 1988). A complete reevaluation of the Lower Colorado alluvial sequence is in progress by the author.

23

Bibliography

Allison, J. E., Dittmar, G. W., and Hensel, J. L. (1975) Soil Survey of Gillespie County, Texas. Soil Conservation Service. United States Department of Agriculture. 80 p.

American Society for Testing Materials (1983) Standard Method for Particle Size Analysis of Soils. Procedures. D-422.

Baker, V. R. (1975) Flood Hazards along the Balcones Escarpment in Central Texas: Alternative Approaches to their Recognition, Mapping, and Management. Geological Circular 75-5. Bureau of Economic Geology, University of Texas at Austin. Austin, Texas. 22 p.

Baker, V. R. (1977) Stream channel respose to floods, with examples from Central Texas. Geological Society of America Bulletin. v.88, pp.l057-1071.

Baker, V. R. (1984) Flood sedimentation in bedrock fluvial systems. in Koster, E. H. and Steel, R. J. (eds.) Sedimentology of Gravels and Conglomerates. Memoir 10, Canadian Society of Petroleum Geologists. Calgary, Alberta. pp. 87-98.

Baker, V. R. and Penteado-Oreliana, M. M. (1977) Adjustment to Late Quaternary climate change by the Colorado River in Central Texas. Journal of Geology. v.85, pp. 395 - 422.

Baker, V. R., Kochel, R. C., Patton, P. C. and Pickup, G. (1983) Paleo-hydrologic analysis of Holocene slack-water flood deposits. in Collinson, J. D. and Lewin, J. (eds.) Modern and Ancient Fluvial Systems. SpeCial Pub!. 6, International Association of Sedirnentologists. London. pp. 229-240.

Barnes, V. E. (1952a) Geology of the Cain City Quadrangle. Bureau of Economic Geology. University of Texas. Austin, Texas.

Barnes, V. E. (1952b) Geology of the Palo Alto Creek Quadrangle. Bureau of Economic Geology. University of Texas. Austin, Texas.

Barnes, V. E. (1966) Geology of the Stonewall Quadrangle. Bureau of Economic Geology. University of Texas. Austin, Texas.

Barnes, V. E. (1981) Geologic Atlas of Texas: The Llano Sheet. Bureau of Economic Geology. University of Texas. Austin, Texas.

Birkeland, P. W. (1984) Soils and Geomorphology. Oxford University Press. New York. 371 p.

Blum, M. D. (1987) Late Quaternary Sedimentation by the Upper Pedernales River, Texas. MA Thesis, Department of Geography, University of Texas at Austin. Austin, Texas.

Blum, M. D. (1989) Geoarchaeology and Quaternary Stratigraphy on the Concho and Upper Colorado Rivers, West Texas. Abstracts with Programs, Southcentral Section of the Geological Society of America. v. 21, p. 4.

Blum, M. D. and Valastro, S. Jr. (in press) Response of the Pedernales River of Central Texas to late Holocene climatic change. Annals of the Association of American Geographers.

Bomar, G. W. (1983) Texas Weather. University of Texas Press. Austin, Texas. 265 p.

Caran, S. C. and Baker, V. R. (1986) Flooding along the Balcones Escarprnent, Central Texas. in Abbott, P. L. and Woodruff, C. M. (eds.) The Balcones Escarpment: Geology, Hydrology, Ecology, and

24

Social Development in Central Texas. Comet Reproduction Services. Sante Fe Springs, California. pp. 1-14.

Carr, J. T. (1967) Climate and Physiography of Texas. Report 53, Texas Water Development Board. Austin, Texas. 27 p.

Dunlap, D. (1983) Quantitative Analysis of the Vegetation of the Texas Hill Country. Unpublished Thesis. University of Texas. Austin, Texas. 82 p.

Dury, G. H. (1965) Theoretical Implications of Underfit Streams. Professional paper 452-C, United States Geological Survey. Washington, D. C.

Ferring, C. R. (1986) Late Quaternary geology of the Upper Trinity River, Texas. Abstracts With Programs. Annual Meeting of the Geological Society of America.

Folk, R. L. (1980) Petrology of Sedimentary Rocks. Hemphill Press. Austin, Texas. 182 p.

Frederick, C. D. (1987) An Investigation Into the Paleoenvironmental History of the Austin Mastodon Siie. -Unpublished Thesis, University of Texas at Austin. Austin, Texas. 65 p.

Gile, L. H., Peterson, F. F., and Grossman, R. B. (1966) Morphological and genetic sequences of carbonate accumulation in desert soils. Soil Science. v. 101, pp. 347-360.

Haas, H., Holliday, V. T., and Stuckenrath, R. (1986) Dating of Holocene stratigraphy with soluble and insoluble organic fractions at the Lubbock Lake Archaeological Site: an ideal case study. Radiocarbon. v. 28, pp. 473-485.

Hall, S. A. (1986) Late Quaternary alluvial chronology of the Southern Plains. Abstracts With Programs. Annual Meeting of the Geological Society of America.

Kier, R. S., Garner, L. E., and Brown, L. F. (1977) Land Resources of Texas. Bureau of Economic Geology, University of Texas at Austin. Austin, Texas.

Kochel, R. C. (1988) Extending stream records with slackwater paleoflood hydrology: examples from West Texas. ill Baker, V. R., Kochel, R. C., and Patton, P. C. (eds.) Flood Geomorphology. John Wiley and Sons. New York. pp. 377-392.

Kochel, R. C. and Baker, V. R. (1988) Paleoflood analysis using slackwater depOSits. ill Baker, V. R., Kochel, R. C., and Patton, P. C. (eds.) Flood Geomorphology. John Wiley and Sons. New York. pp. 357-376.

Kochel, R. C., Baker, V. R., and Patton, P. C. (1982) Paleohydrology of Southwestern Texas. Water Resources Research. v. 18, pp. 1165-1183.

Machette, M. N. (1985) Calcic soils of the southwestern United States. ill Weide, D. L. (ed) Soils and Quaternary geology of the southwestern United States. Special Paper 203, Geological Society of America. pp. 1-21.

Mear, C. E. (1953) Quaternary Geology of the Upper Sabinal River Valley, Texas. MA Thesis, Department of Geological Sciences, University of Texas at Austin. Austin, Texas.

Meltzer, D. J. and Mead, J. I. (1985) Dating late Pleistocene extinctions: theoretical issues, analytical bias, and substantive results. ill Mead, J. I. and Meltzer, D. J. (eds.) Environments and Extinctions: Man in Late Glacial North America. Center for the Study of Early Man, University of Maine at Orono.

25

Orono, Maine. pp. 145-174.

Miall, A. D. (1985) Architectural element analysis: a new method of facies analysis applied to fluvial deposits. Earth-Science Reviews. v. 22, pp. 261-308.

Patton, P. C. and Baker, V. R. (1977) Geomorphic response of Central Texas stream channels to catastrophic rainfall and runoff, in Doering, D. O. (ed.) Geomorphology in Arid regions. Allen and Unwin. Winchester, Mass. pp. 189-217.

Patton, P. C. and Dibble, D. S. (1982) Archaeologic and geomorphic record for the paleohydrologic record of the Pecos River in WestTexas. American Journal of Science. v. 282, pp. 97-121.

Patton, P. C., Baker, V. R., and Kochel, R. C. (1979) Slack water deposits: a geomorphic technique for the interpretation of fluvial paleo- hydrology. in Rhodes, D. D.-and Williams, G. P. (eds) Adjustments of the fluvial system. KendalVHunt Publishing Co. Dubuque, Iowa. pp. 225-253.

Prewitt, E. R. (1981) Archaeological investigations at the Loeve-Fox, Loeve, and Tombstone Bluff Sites in the Granger Lake District of Central Texas. Texas Archaeological Survey, University of Texas at­Austin. Austin, Texas. 369 p.

Singer, M. J. and Janitzky, P. (1986) Field and Laboratory Procedures Used in a Soil Chronosequence Study. Bulletin 1648, United States Geological Survey. Washington, D. C. 49 p.

Shepard, R. G. (1979) River channel and sediment response to bedrock lithology and stream capture, Sandy Creek, Central Texas. in Rhodes, D. D. and Williams, G. P. (eds) Adjustments of the Fluvial System. Kendall/Hunt Publishing Co. Dubuque, Iowa. pp. 255 - 275.

Soil Survey Staff (1975) Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting a Soil Survey. Soil Conservation Service. United States Department of Agriculture. Washington D. C. 754 p.

Sorenson, C. J., Mandel, R. D., and Wallis, J. C. (1976) Changes in bioclimate inferred from paleosols and paleohydrologic evidence in east-central Texas. Journal of Biogeography. v. 3, pp. 141-149.

Stricklin, F. L., Smith, C. I., and Lozo, F. E. (1971) Stratigraphy of Lower Cretaceous Trinity deposits of Central Texas. R171. Bureau of Economic Geology. University of Texas. Austin, Texas. 37 p.

Valastro, S., Davis, E. M., and Rightmire, C. T. (1968) University of Texas Radiocarbon Dates VI. Radiocarbon. v. 10, pp. 384-401.

White, S. E. and Valastro, S. (1981) Pleistocene glaciation of Volcano Ajusco, Central Mexico, and comparison with the Standard Mexican GlaCial Sequence. Quaternary Research .. v. 21, pp. 21-35.

Woodruff, C. M. (1977) Stream piracy near the Balconies Fault Zone, Central Texas. Journal of Geology. v. 85, pp. 483-490.

Woodruff, C. M. and Abbott, P. L. (1979) Drainage basin evolution and aquifer development in a karstic limestone terrain, South-Central Texas, U. S. A. Earth Surface Processes. v.4, pp.319-334.

26

HALL'S CAVE

Rickard S. Toomey III

Dept. of Geological Sciences and Texas Memorial Museum, University of Texas, Austin, Tx 78713

INTRODUCTION

During the last 15,000 years many floral, faunal, climatic, and geomorphic changes have occurred in North America. Because central Texas is located at the boundary of several biological and geological provinces, the deposits in this area are especially useful in studying these changes in south-central North America. A paleoenvironmentally focused study of Hall's Cave (Kerr County, Texas) provides an excellent opportunity to study the changes that occurred during the transition from Pleistocene to Holocene and throughout the Holocene.

Field work in the early 1970s, as well as my work beginning in 1986, has established that Hall's Cave contains a well- stratified clastic deposit spanning the Pleistocene-Holocene transition. In addition to clastic sediments, the deposit also contains vertebrate remains, pollen, and snails. Hall's Cave contains the most complete Late Pleistocene through Holocerie sequence known from one place in central Texas and one of the most complete known in the south-central United States. The presence of vertebrates, suriace derived clay minerals, and pollen is especially important. The on-going integrated study of the vertebrate remains, pollen, clay mineralogy, and sediments within a stratigraphic sequence, controlled chronometrically with radiocarbon dates, will allow a comprehensive paleoenvironmental reconstruction of central Texas environments in the latest Pleistocene, Pleistocene-Holocene transition and Holocene.

GEOGRAPHIC AND GEOLOGIC SETTING

Hall's Cave (Texas Memorial Museum locality number 41229) is located In western Kerr County (Figure 1 in the introduction to this guidebook), approximately 50 kilometers west-northwest of Kerrville. It is on the T. D. Hall Ranch in Mountain Home, Texas.

Hall's Cave currently consists of a single large room which is entered by walking down a talus slope at the bottom of a small sinkhole (Figure 1). The walk-in entrance is the only known opening large enough to allow the entrance of vertebrates. Before the early 1930s, several additional rooms were accessible from the main room. No other entrances are associated with these rooms. These rooms were unfortunately closed off because of the hazard they posed to the goats which were wintered in the cave. Subsequently the passages to these rooms were lost (T.D. Hall, 1986, personal communication).

Hall's Cave is in the center of the Edwards Plateau. Over 75 percent of the known caves in Texas occur within the nearly flat-lying Cretaceous limestones of this geomorphic region or within its westward extension, the Stockton Plateau (Kastning, 1983). They are not, however, evenly distributed across the region. The distribution of the caves is controlled by variation in such factors as lithology, fracturing, local and regional dip, and topography (Kastning, 1983). The Edwards Group was subject to karst activity during the Late Cretaceous, the Miocene, and during the Quaternary; together these episodes formed the large karst terrain (Kastning, 1986).

The cave has formed within the lower portion of the Lower Cretaceous Segovia Formation of the Edwards Group (Barnes, 1981). Joint control may be a factor in the formation of this cave; evidence for this includes linear speleothem traces on the ceiling.

Speleothems, especially stalactites and draperies, are abundant on the walls and ceiling of the cave. They are especially common in the rear portion of the cave. Most of these speleothems are inactive. The rear of the cave contain a higher proportion of active speleothems than does the section near the entrance. Several rimstone dams are present in the rear of the cave. These rimstone dams have a total relief of approximately 20 cm. These indicate ponded water in this area of the cave to a depth of at least 20 cm. No ponded water occurs in the cave today.

1

~o ') Sl !/ ,,"0 ... o

C.f: ::T

~~ /J II)

:J

01 CO

c .. " ~

\ ( Rockfall

31 /'~.

Figure 1 -- Map of Hall's Cave, Kerr County Texas. This map is based on data from plane table survey and brunton and tape measurements. Composite Pit I is represented on the map as C.P. I

Most of the floor of the cave is covered by clastic sediments and modern guano. A very extensive rockfall covers much of the southern and southeastern portions of the cave. This rockfall may be a single fall or more likely is a composite of several falls. Another smaller rockfall is associated with the cave entrance.

MODERN ECOLOGICAL SETTING

VERTEBRATE FAUNA

Hall's Cave is located in the center of the Balconian biotic province (Blair, 1950). The Balconian Province includes all of the Edwards Plateau region. The Balconian vertebrate biota is a unique mixture of animals found in the biotic provinces surrounding it: the Kansan, Texan, Austroriparian, Tamaulipan, and Chihuahuan biotic provinces. Of about 405 terrestrial vertebrate species (Davis, 1974; Dixon, 1987; Wolfe, 1965) found on the Edwards Plateau only seven species of salamanders are endemic (Dixon, 1987). The lack of endemism in the Balconian fauna may contribute to the utility of Edwards Plateau faunas and paleofaunas in reconstructing past environments. Because the modern fauna is a mixture of various elements from very different climatic settings, e.g. the Chihuahuan province in an arid climatic regime and the Austroriparian from a moist to sub-humid regime (Blair, 1950), the changes in composition of the mixture will in part reflect changes in climate through time.

PALEOENVIRONMENTAL ANALYSIS IN CENTRAL TEXAS

Since 1957 much research has been undertaken to reconstruct the Late Pleistocene and Holocene environments of central Texas. Today this area has one of the best chronological records of Late Pleistocene and Holocene faunal and environmental changes in the country (Graham et al., 1987). Paleoenvironmental research has proceeded along many lines. Figure 2 shows some of the sites important in the investigation of the Late Pleistocene and Holocene vertebrate paleontology and paleoenvironments of central Texas. Although most of this research has been in vertebrate paleontology, research in palynology, clay mineralogy and sedimentology of cave deposits, geomorphology, and terrestrial molluscs has also contributed important paleoenvironmental information.

Some of the most important studies of latest Pleistocene vertebrate faunas in the south-central United States have been on caves in the Edwards Plateau region of Texas. The earliest systematic study of one of these caves was the analysis of fauna from Longhorn Caverns by Semken (1961). Schulze Cave, which has a diverse Pleistocene fauna (Dalquest, et al. ,1969) is within 30 miles of Hall's Cave. The fauna from Cave-Without-A-Name, although largely unstudied, is another important central Texas cave fauna (Lundelius, 1967). Friesenhahn Cave (Graham, 1976) is another important site for understanding Late Pleistocene and Early Holocene environments in the region. The main limitation of these sites is that each contains only limited time sequences rather than a well stratified sequence through the Pleistocene and Holocene.

PREVIOUS INVESTIGATIONS

In the past twenty years investigation of Hall's Cave has taken place in three phases. The first phase was the Klein Cave phase; the second phase was Michael B. Collins' wort< on Hall's Cave. The third phase is the present work (including a brief Investigation by Robert S. Rosenberg).

KLEIN CAVE PHASE

From 1966 to 1968 Edward Roth worked on a cave, which he named Klein Cave. This wort< was for a Master's degree in the Department of Biology at Midwestern State University. He wet- screened 1500 pounds of matrix and analyzed the resultant fauna (Roth, 1972). The cave which. Roth worked is certainly Hall's Cave (Michael Collins and T.D. Hall, 1986, personal communication; E. L. Roth, 1989, personal communication). I have been unable to locate his excavation, which was apparently along the south wall of the cave. The fauna that Roth (1972) reports agrees well with my own and with that from other central Texas sites; however, the cave stratigraphy which he describes (Roth, 1968) does not agree with the stratigraphy of Composite Pit I.

3

Figure 2a -- List of some of the many sites in central Texas which provide paleoenvironmental or paleontologic information. These sites are taken from a wide variety of literature sources and from Vertebrate Paleontology Laboratory records.

Vertebrate Faunal Sites • Halls Cave A Angelina Cave B Barton Road Site Bf Bonfire Shelter Bk Baker Cave Bu Burial Cave C Clamp Cave Cc Cascade Caverns Cp Centipede Cave Os Coontail Spin Cave D Deep Cave Dm Damp Cave Dv Devil's Mouth Ow Don WilliaJIIB Cave E Eagle Cave F Friesenhahn Cave F1 Felton Cave Fr Fern Cave Fs Fallen Stalagmite Cave H Hinds Cave Hc Honey Creek Cave Hz Hitzieldenl Cave I Ireland's Cave K Kincaid Shelter L Laubach Cave Ln Longhorn Caverns Lv Levi Rockshelter M Miller Cave Mc Mac's Cave MIn McMin Cave Mn Monte.l.l Rockshelter Mq Mosquito Cave N Natural Bridge Caverns o Oblate Shelter Pw Powell's Cave Q Cueva Quebrada R Rattlesnake Cave S Schulze Cave Sl Saltillo Cave Sm Spider Mountain Cave Sr Smith Rock Sa Seminole Sink W Cave Without A Name WmKerr Wildlife Management Area #1

4

Wn Wunderlich X Cave X Z Ze8ch Cave Zp Zopilote

Pollen Sites Bf Bonfire Shelter Bk Baker Cave Br Boriack Bog Cp Centipede Cave Dm Damp Cave Dv Devil'a Mouth E Eagle Cave F Friesenhahn Cave H Hinds Cave HI Henlhop Bog Lv Levi Hochhelter Pt Patshke Bog Sj Soefje Bog (and South Soefje Bog)

Molluscan Fauna So Smythe's Crossing

Clay Mineralogy and Sedimentology F Friesenhahn Cave Fl Felton Cave

Geomorpology P PedernaJea River Te~ N Natural Bridge Caverns

Figure 2b -- Map showing sites listed in Figure 2a

o 10 1:0 .to to "' , ,! !

MICHAEL B. COLLINS

From 1968 to 1970 Michael B. Collins, of the Department of Geological Sciences, University of Texas at Austin, excavated Hall's Cave. He is the person who gave the cave the name which I have chosen to use. He excavated three areas of the cave: two pits on the main flat and the bear pit (Figure 1). His main pit, Pit I, extended to a depth of 195cm. The material and notes from this excavation are housed at the Vertebrate Paleontology Lab of the Texas Memorial Museum and are integrated with the results of the current work

CURRENT WORK

The current phase of Hall's Cave research began in the summer of 1986. Since that time I have excavated approximately 16 cubic meters of Hall's Cave sediment. Most of this has been excavated in 5 cm levels. The result of this excavation is Composite Pit I, which includes part of Mike Collins' Pit I. This pit has been excavated to the bedrock floor of the cave. The material recovered in the excavation was then washed through a 2mm mesh screen in order to recover bones, snails, seeds, charcoal and any other important remains, which were then picked from the concentrate, identified and catalogued. This phase of the work is continuing.

RESULTS OF CURRENT WORK

Stratigraphy

The Hall's Cave deposit, as revealed by Composite Pit I, consists of six major stratigraphic units (Figure 3). From the bottom these units are the bedrock, the residuum, the travertine wedge, the red clay, the brown Clay, and the dark brown to black unit.

BEDROCK

The lowest unit exposed in Composite Pit I is the cave floor which is composed of limestone of the lower portion of the Segovia Member of the Edwards Limestone. The lower portion of the Segovia Member is a "light-yellowish-gray miliolid limestone amd marl and marly limestone" (Barnes, 1982, p. 4). The floor of the cave is irregular in the pit. For this reason the Segovia is only exposed in the northwestern portion of Composite Pit I. In this portion of the pit it is exposed below approximately 2 meters.

RESIDUUM

The "Residuum" is an approximately 15 cm thick layer of poorly consolidated, weakly cemented and uncemented, sandy sediment, composed mainly of quartz sand and box-work dolomite rhombs. This unit is interpreted to be the insoluble residue remaining when the calcite of the Segovia Member dissolved forming Hall's Cave. Only about 2-3 percent insoluble material would be required to fOrm a residuum layer the thickness found in the cave. This percentage is in the ranges found for the Edwards limestones by Rodda and others (1966). The presence of this unit indicates that the Segovia limestone below is bedrock rather than a large block spalled from the cave roof. This unit is exposed in the northwestern wall of Composite Pit I and follows the irregularities of the floor across the pit to form the floor of much of the southeastern half of the pit.

TRAVERTINE WEDGE

The travertine wedge is a large coherent mass of travertine which extends across Composite Pit I. The top of the travertine wedge dips to the southeast within the pit. This is due both to the irregularity of the underlying bedrock floor and residuum and to the thinning of the travertine wedge to the southeast.

In the southwestern wall of Composite Pit I two travertines are visible. By tracing the travertine wedge, red clay and residuum around the southern corner of the pit, one can see that the lower travertine unit in the southwestern wall is the same unit as the travertine wedge. The upper travertine unit was deposited while the red clay was being deposited in the rest of the pit. The deposition of the stalagmite in the western corner of CompOSite Pit I may be associated with the end of deposition of the upper unit.

6

E N

NW

Dark Brown to Black Unit

... lx-6097 2,490 ± 90

Brown Clay

lx-6154 7,940 ± 130

===-Travertine Stringers

Red Clay

wedge lx-6096 12,470 ± 160

20 cm I Tx-6137 13,050 ± 150 ,.-,~ ~

~e~ <lCe

Bedrock

FIgure 3 -. General idealized sketch of the northeastern portion of Composite Pit I. The northeastern wall is visible as well as part of the northwestern wall and sectioned travertine wedge. The three clastic units are visible in beth walls. The location of radiocarbcn dates is also shown in the sketch.

7

The travertine wedge definitely formed in vadose conditions after the water table dropped below the level of the cave. Stratigraphic indications in Composite Pit I suggest that it may have been deposited before a major entrance formed. A radiocarbon date on the outermost one-half cm of the travertine at approximately 180 cm indicates that the travertine wedge had largely formed by about 13,000 years BP (Tx-6137, 13,050±140 yrs BP, corrected for 13C). Because red clay sediments rest on the travertine wedge this date is a minimum approximation of the openning of the Hall's Cave to clastic sediments.

RED CLAY

The red clay is composed of sticky red clay with some cobbly layers and some boulders. The sediment varies from dusky red (10R 3/3) to red (2.5YR 4/6). The yellower color at the bottom may be due to the incorporation of travertine from the wedge into the sediment or due to primary changes in sediment. The widespread occurrence of these red clays in central Texas caves and their relatively uniform nature suggests to me. that the red color of the clay is probably the result of the erosion of red sediment (presumably a red soil horizon) rather than the result of post-depositional changes.

The red clay contains some large boulders (up to approximately 100 kg) that are probably the result of ceiling spall. In addition some layers are cobbly. The layers from about 235 cm to 220 cm contain large numbers of cobbles including many pieces of cave coral/cave popcorn from the roof of the cave. Large numbers of limestone cobbles also occur in the layers near 200 cm. .

Several travertine stringers occur within the red clay layer; the most notable are at 170 cm and 190 cm. The travertine stringer at approximately 190 cm has a radiocarbon age of 12,470±160 (Tx- 6096, corrected for 13C).

The top of the red clay is at approximately 155 cm. It is weakly cemented with calcite cement for approximately the top 3 centimeters. This carbonate cement at the top of the red clay has an age of 7,940±130 yrs BP (Tx-6154, corrected for 13C).

The date on the top of the red clay is important for several reasons. Many of the caves in central Texas have red clays associated with Pleistocene faunas; these include Longhorn Caverns (Semken, 1961), Freisenhahn Cave (Graham, 1976) and Cave-Without-A-Name. This has led to the speculation that red clay may indicate Pleistocene deposits. The radiocarbon date from Hall's Cave demonstrates that the red clay can extend into the early Holocene. The termination of deposition of red clay in caves on the Edwards Plateau is probably time transgressive.

The cessation of deposition of the red clay in caves on the Edwards may appear time transgressive for at least three reasons. The cessation may be synchronous across Edwards Plateau, but it appears transgressive because all of the caves do not have deposits of the appropriate age. The cessation may be transgressive because the formation of an appropriate soil from which to derive the sediments end~d asynchronously across the Edwards. The cessation of deposition may be time transgressive because reserves of soil produced red sediment were exhausted at different times at different places on the Edwards Plateau. The third option seems to be the most likely, but it does not preclude the second also occurring.

BROWN CLAY

The brown clay extends from about 155 cm to about 80 cm. A small swale is present at the top of the brown clay in the northeast wall of Composne Pit I. The brown clay is a reddish dark brown (5YR 3/3) silty clay which becomes more pebbly below 1 m. The lowest levels of the brown clay 140-155 cm where probably deposited at a low sedimentation rate. Evidence for this includes the extremely bone rich sediment (up to nearly 50% small mammal bone by volume), the presence of several dish like stalagmites, and small travertine cemented masses in these levels. These layers are the most bone rich layers in Hall's Cave. The source of the brown clay is surface sediments near the cave entrance.

A small hearth (approximately 30 cm in diameter and 2 cm thick) is present in the southwestern wall of Composite Pit I at about the 100 cm level. Charcoal from this hearth has been submitted for radiocarbon dating.

8

DARK BROWN TO BLACK UNIT

The Dark Brown to Black Unit, 80 cm to top, is quite rich in organic sediments. The color of this unit varies from very dark brown (1 OYR 212) to black (1 OYR 211). The lower boundry of this unit is not flat; a large 10 cm deep swale is clearly visible in the northeastem wall of the pit and other smaller irregularities are also present. The top 10 cm contains abundant quano (bat, bird and goat), feathers, etc. Large wood fragments are found in this unit. In general, the unit is silly to clayey. However, the layers from 15 - 25 cm are cobbly. Burnt cobbles occur commonly above about 30 cm. Burnt cobbles are present below this level but are less common. These cobbles have probably been washed in from the burnt rock midden which is present on the up-slope rim of the sinkhole (see discussion of archaeology presented in "Stops" section below). Based on the color of the sediments, the organic content of the sediment decreases with depth. The sediments of the top 20 cm are probably largly derived from the topSOil lost off the surrounding Edwards Plateau due to historic anthropogenic soil stripping. One radiocarbon determination is available from the dark brown to black layer. Charcoal collected from 45-50 cm has a radiocarbon age of 2,490±90 yrs BP (Tx-6097).

Vertebrate Paleontology

The Hall's Cave deposit is extremely rich in vertebrate remains. Although the concentration of fossils varies within the deposit, no layer can be considered sterile. This is one of the most important aspects of the deposit. Bone is especially abundant in the levels at 45 - 55 cm, 80 - 100 cm, 140 - 160 cm and 230-240. The layers in· the 140 - 160 cm range are extremely rich in bone. Within these levels, in addition to bone, abundant coprolites are present, which have been tentatively identified as having been produced by coyotes (Canis latrans).

The taphonomy of the deposit is important, because taphonomy affects the paleoenvironmental interpretation. At least four different sources probably contributed vertebrates to the deposit. These sources are 1) animals that lived and died in the cave, 2) animals that washed into the cave, 3) animals that were brought in by mammalian carnivores, and 4) animals that were eaten by raptors. Although it is impossible to fully separate these sources, the second source is probably the least important factor in the accumulation of the Hall's Cave deposit.

Animals that lived and died in the cave certainly contributed to the deposit. At least 10 different species of vertebrates were observed using the cave during excavations (see discussion below). Animals whose remains are found in the cave which probably lived in the cave include many bats, some carnivores, frogs and some birds.

Another component of the fauna is derived from the remains of carnivore meals. This is definitely a factor In the Hall's Cave deposit. The coprolites from approximately 145 cm provide ample evidence of the contribution of carnivore meals.

Raptor (probably owl) pellet remains are also a significant contributer to the vertebrate remains. The abundance of bones of small mammals (juvenile rabbit and smaller), the completeness of long bones, and the relative abundance of various skeletal elements are important in identifying owl pellet accumulations. These characteristics, as well as the presence of some skull fragments in which the back half of the skull is missing, are characteristic of owl pellets (Dodson and Wexlar, 1979). In addition, a few masses which appear to be remains of owl pellets are found in the brown clay at about 145 cm.

As of March 1989 over 5200 vertebrate specimens had been identified and catalogued. These specimens represent over 90 taxa, of which approximately 60 are mammalian (Table 1 and Figure 4). In addition to containing remains of many of the vertebrates found near t.he cave today, the Hall's Cave fauna contains both extinct taxa and extralimital taxa (animals no longer found in the area today). Extinct taxa which have been recovered at Hall's Cave include flat-headed peccary (Platygonus compressus), giant bison (probably Bison antiquus), horse (Equus sp.), llama (Hemiauchenia sp. or Paleolama sp.), western vulture (Coragyps occidentalis), and giant tortoise (Geochelone wi/som). The stratigraphic occurrences of many Hall's Cave taxa in Composite Pit I are shown in Figure 4.

9

Table 1 -- Non-mammalian fauna of Hall's Cave. The mammalian fauna is in Figure 4. Both lists are preliminary and subject to revisions and addijlons. They are considered corect as of March 16, 1989. All animals do not occur throughout the Hall's Cave section.

Ostelchthys Lepidosteidae

Lepidosteus sp. Ictaluridae

IctalufUs ct. I. furcatus Centrarchidae

Unid. Centrarchid Amphibia

Anura Unid. Ranidae Unid. Bufonidae

Caudata

Reptilia

Unid Caudata Ambystoma sp.

Emydidae Indet. aquatic Emydldae Terrapene carolinensis Terrapene sp.

Testudidae Geochelone wilsoni

Chelydridae prob. Chelydra serpentina

Iquanidae indet. Sceloporinae Phrynosoma cornutum Phrynosoma douglassi

Crotalidae Unid. Crotalidae Crotalus sp.

Colubridae Unid Colubridae

Aves Anatidae

Unid. teal sized Anatidae Cathartidae

Coragyps occidentalis Accipijeridae

Unid. AccipHeridae Meleagridae

Meleagris gallopavo Phasianidae

Colinus virginianus Tetraonidae

Unid. Tetraonidae Columbidae

Unid. Columbidae Columbidae cf. Zenaidura macroura and Zenaida asiatica Columbidae ct. Zenaidura macroura

Charadrmormes Unid. Charadrmormes

Strigidae Asio sp.

10

Aves (con't) Cuculiformes

Coccyzus sp. Geococcyx sp.

Picidae Colaptes auratus cf. Colaptes auratus

Passeriformes Unid. Passeriformes

CONidae CONUSSp.

Hirundinae Petrochelldon fulva

Fringillidae Icteridae

cf. Quiscalus sp. Unid. Icteridae

Figure 4 -- Stratigraphic diS1ribution of selected Hall's Cave taxa wijhin Composije Pij I. These data are preliminary and subject to revision. The chart is considered correct as of March 16, 1989. The notations in the status column are as follows: M -- Hairs cave is wijhin the animals modem range; 0 - Hall's Cave is outside of the range of the species; E --the species is extinct.

IA - .. matldae

AMPHIB Ambysto

Amb ystoma sp.

A- -.. e

.. .. REPTILI

Iquanlda Phyr nosoma douglassi

nosoma cornutum e rapene sp.

.. .. .. .. ..

Phry Emydida

Ter AVES .. Cathartid

Co

.. .. .. .. .. .. ae

ragyps occidentalls Piddae

Co laptes auratus

LIA .. .. ae

.. MAMMA Didelphld

Old elphls vlrglniana

ex cinereuslhaydeni ptotls parva

Sorieidae Sor Cry Bla Not

rlna sp. losorex crawfordl

lopus aquatlcus Talpidae

Sca Homonid

Ho ae rna sapiens IIonldae strellus subllavus strellus hesperus slcus fuscus eelus humeralls tis lucifugus tis griscescens tis keenl tis veHfer

..

Vespertil Plpl Pipl Epte Nyct Myo Myo Myo Myo Las Las

lurus bolreallslseminolus lurus cinereus

Molossld Tad

ae anda brasiliensis

nis latrans Canidae

Ca Ca Ca Uro

nls sp. (small) nis lupus cyon or Vulpes da Procyon I

Pro Ursidae

Urs Urs

cyon lotor

us americanus us sp.

1 1

.. .

I- ... ... 10 .. I- .. 1-- -- • 0

i" --.. -.. . .. 10 .. 1--0 I-M cf 0 -I- .. . .. I- .. I- .. .. .. .. .. E -M -l-

'" .. .. .. .. .. .. _. .. I- -

M-

0 1-- ... 0 0 M

0

M I--M 0 --0 M P-0 r 0 I-cr-0 Cr M

P- - .. rc M -M

MI-- 1-: ~ -o _ I-- .. I-- I-> MI- - 1---MI-- p..-

0 I--0 -

Figure 4 (con't) -- Stratigraphic occurrence of selected taxa within Hall's Cave Composne Pn L

era onca rufus

Felidae Panth Fells Felis weldil or yaguoaroundi

e ala frenata or erminea

Mustelida Must Meph Cone Spilog

itis mephitis patus mescleucus ale putor1us

e Leporida Sylvi Lepu

lagus sp. ssp.

rus niger us sp.

Sciuridae Sciu Sciur Sper mophilus spilosoma or

tridecimllneatus Cyno mys sp.

ae ys sp. omys sp.

Geomyld Geom Thom Papp ogeomys castanops

idae todipus hlspidus

Heteromy Chae Perc Dlpod

gnathus sp. (small) omys elator

rodontomys sp. myscus sp.

ys taylorl homys leucogaster

Cricetidae Reith Pera Balom Onyc Neot Sigmod Micro Micro Micro Syna

oma sp. on hlspidus

tus pennsylvanicus tus ochrogaster tus plnetorum ptomys cooperl ae

gonus compressus Tayassuld

Platy Cervidae

Odoco Bovidae

Biso Camelida

Pale Equidae

Equu

Ileus sp.

n antlquus e

olama or Hemlauchenia

ssp.

12

0 p.- .. M faa 0 ..

MO -M ~ -M -M

M M -M -.. 0 0 0 --0 -0 0 ... M M .. 0

M

M ..- II1II

M M M 0 -0 M~

0 -.. E -M - - ... E .. E -E ~

preliminary Paleoenvironmental Interpretations

The extralimital species in a deposit often provide much of the information on changing environments. This is due to two important factors. The first is that the presence of species that occur in the area today does not indicate changes in environment and the absence of these species may be due to factors other than environmental changes. The second is that we know less about the environmental requirements of extinct taxa than we do about those of extant ones.

The appearances and disappearances of taxa at various levels indicate both general environmental conditions and the changes which have occurred in these conditions. Some preliminary tentative conclusions concerning the Hall's Cave fauna and its relationship to paleoenvironments are presented below.

The presence of horse (Equus sp.), bison (Bison antiquus), prairie dog (Cynomys sp.), and meadow vole (Microtus pennsy/vanicus) in the latest Pleistocene deposits, i.e. below approximately 175 cm, indicates the presence of rather extensive areas of grassland in the vicinity of the cave. Bog lemming (Synaptomys coopen) remains indicate permanent standing water or denser groW1h meadow nearby (Burt and Grossenheider, 1976). The presence of flickers (Co/aptes auratus) indicate that large trees were present; however, the trees may not have been in the immediate vicinity of the cave. Masked shrews (Sorex cinereus), a short-horned horned lizard (Phrynosoma doug/assi) and Microtus pennsy/vanicus are clear indicators of cooler temperatures, at least in the summer. The presence of short-tailed shrews (Blarina sp.) and Synaptomys cooperi probably indicate greater effective moisture.

The disappearance of the big brown bat (Eptesicus ruscus) from the Hall's Cave deposit at approximately 170 cm is one of the most interesting and, perhaps, informative faunal changes. Eptesicus is not found on the Edwards Plateau today (Manning, et a/., 1989). McNab (1974) calculated that a bat the size of Eptesicus can only winter in a cave if the cave microclimate is less than 7°C. He also notes that clustering of Eptesicus in caves only occurs at less than 5°C. This agrees with the observation that Eptesicus only uses caves as winter roosts from central Kentucky northward. Below 220 cm Eptesicus is abundant in Hail's Cave sediments, suggesting clustering and, thus, winter cave microclimates below 5°C. From 220-170 cm Eptesicus occurs in much smaller numbers; this may indicate solitary hibernating and winter microclimates between 5°C and 7°C. Above 170 they do not occur which may indicate that cave microclimates rose to higher than 7°C. On February 26, 1989 cave wall temperatures in Hall's Cave were 11°C.

One of the most common features of late Pleistocene faunas is the presence of "disharmonious" assemblages (Lundelius, et a/., 1983). Disharmonious assemblages are groupings of species that "today are separated geographically and appear ecologically incompatible" (Lundelius, et aI., 1983, p. 346). These communities represent individualistic responses to a climate which was probably more equable (Graham and Lundelius, 1984). Examples of disharmonious pairs within the Hall's Cave fauna (baesd on ranges in Hall, 1981) include the following: Notiosorex crawtordi and Synaptomys cooperi (level 195-200 cm); Myotis velifer and Sorex cinereus (levels 190-195 cm, 220-225 cm, and 250-255 cm); and Panthera onca and Microtus ochrogaster (level 175-180 cm).

During the Holocene several faunal changes occur which indicate environmental changes. These events cannot be more precisely placed within the Holocene until more radiocarbon dates become available. The extirpation of both moles (Sca/opus aquaticus) and short-tailed shrews (Blarina sp.) probably resulted from climatic change. The most likely cause seems to be decreasing soil moisture, which could be due to changes in preCipitation, temperature, or both.

Semken (1961) suggested that the disappearance of the geomyids (Geomys sp., Thamamys sp., and Pappogeomys sp.) from the Edwards Plateau was a result of anthropogenic- induced soil stripping caused by over-grazing during the last 150 years. The late disappearance of geomyids at Hall's Cave supports this conclusion. The presence of Geomys sp. indicates several characteristic of the soils which could have been found in the vicinity of Hail's Cave before this soil stripping ( Davis et a/., 1938; Downhower and Hall, 1966). There must have been soils in the vicinity of the cave which had an at least 10 cm thick (and probably greater than 15 cm thick) surface horizon. These soils probably also had between 10 and 30 percent clay and between 40 and 90 percent sand and were well drained. The least shrew (Cryptatis parva) may also have been a victim of this soil stripping.

13

CONCLUSIONS

The work at Hall's Cave is continuing actively. The analysis of the vertebrate faunas is far from complete. I am continuing to study the vertebrate material, as well as, the sediments. Several intriguing questions have arisen recently. Why are microtines rare to absent in the levels between 40 cm and 90 cm? Why are free-tailed bats missing from Hall's Cave deposites and from those of other caves in Texas and New Mexico? The answers to these questions and others like them may provide interesting information on central Texas paleoenvironments.

As analysis of the Hall's Cave sediments continues, reconstruction of the environments of central Texas during the past 13,000 years will improve. Pollen analysis and mineralogical studies will provide additional data bearing on the environments of the latest Pleistocene and Holocene. Because the deposits in Hall's Cave are well stratified and reasonably complete, they provide a unique opportunity to understand the dynamics of the faunal change associated which the end of the last glaciation.

14

I' I

Roadlog to Hall's Cave For Sunday Apr. 9, 1989; From Save Inn, Fredrlcksburg, Texas

Milage

O.Omi

0.5mi

Feature or Direction

Save Inn, Fredricksburg Texas -- Turn right on US 290

Turn left on Tx 16 S (Traffic ight)

3.9 mi On left as you pass the entrance to Lady Bird Johnson Municipal Park, notice the

23.2mi

paleosol developed into a Unit A, Early to Middle Pleistocene terrace remnant of the Pede males River or Live Oak Creek.

Enter IH-10 going west

During the next 18 miles you will be passing through several roadcuts these are in rocks of the Edwards Group. The Edwards Group in this area consists of about 350 ft of Lower Cretaceous limestones and dolomites which were deposited during the Albian on a flat shallow platform known as the Comanche shelf (Rose, 1972). These rocks are almost flat-lying and virtually undeformed tectonically. The folds and brecciation that can be seen in some of ihe roadcuts is due to karstification and the dissolution of evaporites. Gypsum within several horizons within the Kirschberg Evaporites has dissolved leaving the solution-collapse breccia seen commonly in the roadcuts along this stretch of IH-10 (Kastning, 1987). The timing of the dissolution of the Kirschberg Evaporite has not been reliably established; however, karstification of the Edwards Group has been occurring since Late Cretaceous times.

31.8 ml . In the roadcut at the 500 mile marker on West IH-10 you can see a cave passage filled with red clay. This small passage (with a circular cross-section) is exposed on both sides of the highway approximately 10ft. above the base of the roadcut. A sample of this clay was screened for vertebrate remains and contained none (snails were also lacking). These clay filled cave passages are very common in the Edwards Group limestones and occasionally they produce Pleistocene vertebrate faunas.

41.2 mi Exit IH-10 on TX 41 (Mt. Home and Rocksprings Exit). Proceed west on Tx 41 (the only direction it goes). You will cross TX 27 at Mt. Home. Continue west on Tx 41.

55.1 mi Tum into the T.D. Hall Ranch (left).

15

"STOPS"

Around the Cave

SOILS

The soils in the area around Hall's Cave are all very thin. These soils are of the Tarrant-Eckrant association (Dittemore and Coburn, 1986). The Tarrant and Eckrant are classified as clayey-skeletal, montmorillonitic, thermic lithic Haplustolls and Calciustolls. Today, in general, the soils near the cave are extremely rocky and have a total depth of less than 30 cm.

VEGETATION

The most characteristic floral assemblages of the Edwards Plateau are the Live Oak-Ashe Juniper Parks and Woods and Live Oak-Mesquite-Ashe Juniper Parks (McMahan, et al., 1984). Like the vertebrate assemblage, the floral assemblages are a mixture of species with wider distributions.

Although the changes in the composition of the flora appear to have been relatively minor during the last 100 years, the relative abundance of the floral elements probably has been significantly altered. Before the middle of the 1800s the flora of the Edwards consisted of an open grassland with fewer trees (Weniger, 1988). The suppression of natural wildfires by European immigrants has led to this increase in arboreal species (most notably Ashe juniper, Juniperus ashe/) (Texas Parks and Wildlife Department, 1986).

FAUNA

Many vertebrates are found in and around Hall's Cave today. Mexican freetall bats (Tadarlda brasiliensis), cave bats (Myotis velitet'), opossums (Didelphis virginiana) , hog-nosed skunks (Conepatus mesoleucus), cave swallows (Petrochelidon tulva), the Texas blind snake (Leptotyphlops dulcis), and leopard frogs (Rana berlandien) were all found living within the cave during field work (summer 1987,1988). A great horned owl (Bubo virginianus) also frequents the cave; this is important in that it demonstrates that owls will frequent this cave and supports the idea that much of the small bone in the deposit could be from owl pellets. Many other species occur in the area immediately around the cave. Mammals living around the cave include deer (Odocoileus virginianus) , rock squirrel (Spermophilus variegatus), jackrabbit (Lepus calitornicus), and raccoon (Procyon lotot'). Reptiles found near the cave include western diamondback (Crotalus atrox), Coachwhip snake (Masticophis flagellum), whiptail (Cnemidophorus gularis), greater earless lizard (Holbrookia texana) and tree lizards (Urosaurus ornatus). Several anurans are also found around the cave; these include spadefoot toads (Scaphiopus couchi ), toads (Buro valliceps and Buto speciosus) and cliff frogs (Syrrhophus marnockt).

T.D. Hall raises several types of exotic artiodactyls for hunting. On the way to and from the cave you may see axis deer (Cervus axis, from India), sika deer (Cervus nippon, from the Orient), fallow deer (Cervus dama, from Eurasia), and blackbuck antelope (Anti/ope cervicapra, from India), as well as, native whITe-tailed deer (Odocoileus virginianus. Native deer and exotic animal hunting leases are an important source of income in central Texas.

NEAREST PERMANENT WATER

The nearest permanent sources of water at Hall's Cave today are several springs at or near the permanent headwaters of the North Fork of the Guadalupe River. Among these springs are several at the Kerr Wildlife Management Area and Boneyard Spring. These springs are approximately 5 1/2 miles south and southeast of Hall's Cave. This is the direction in which most of the bats from Hall's Cave go each night.

ARCHAEOLOGY

Leland Bement of the Texas Archeological Research Laboratory visited Hall's Cave on March 15, 1989 and provided the following observations. An arcuate burnt rock midden is visible at the upslope rim of the sinkhole. This midden is probably associated with primary cooking and discard activities of people using the cave as a sheltered sleeping area. Burnt rock middens and occupations at caves similar to Hall's Cave are common in central Texas. The burnt rock midden probably dates from between the end of the

16

Early Archaic (ca. 5000 yrs. B.P.) and the end of the Middle Archaic (ca. 2250 yrs. B.P.), although some people suggest that middens such as these may have been used even later. Chert flakes, partial projectile points, and other tools are extremely commoncan on the surface around Hall's Cave.

The Entrance

Because the entrance is a cave's contact with the outside environment, it is important to take the entrance into account when studing any cave. The entrance controls what kinds of conclusions can be drawn concerning sediments and faunal remains. Note the large size of the cave entrance today. This entrance would not restrict the movement of many animals found in North America today. Also note that entrance to the cave is gained by walking down a relatively gentle talus slope. This cave is by no means a trap cave situation; this means that the fauna found in it is definitely not a random sample of the surface fauna.

Note the hackberry tree (Celtis occidentalis) which is growing in the entrance of Hall's Cave. In the Kerr County area caves can often be identified by the presence of hackberry trees in their entrances (D. Harmel and W. Armstrong, 1987, personal communication). The hackberry trees may grow in cave entrances because hackberry trees have higher moisture requirements than other trees in the area. Hackberry seeds are also found in the sediments in Hall's Cave from the Pleistocene levels to the most recent levels.

General Cave Observations

There are several important things to note as you enter the cave.

Notice the cave swallow (Petrochelidon fulva) nests on the walls in the front half of the cave. Hall's Cave is a breeding colony of several hundred cave swallows. These cave swallows produced two clutches of young each of the two summers I was excavating. This is typical of cave nesting cave swallows in Texas (Martin, 1981).

Most of the large amount of guano on the floor of the cave was produced by a Mexican free-tail bat colony (Tadarida brasiliensis mexicana) which inhabits the cave during the summers. Tadarida brasiliensis mexicana is the most common bat on·the Edwards Plateau (Manning, et a/., 1987). During the summer of 1987, the colony reached an estimated size of several hundred thousand. All individuals captured have been males indicating that Hall's Cave is a bachelor colony. Cave swallows and domestic goats (Capra hircus) also contribute to the guano formation.

As discussed above, inactive speleothems, especially stalactites and draperies, adorn much of Hall's Cave. You may wish to venture to the back of the cave to see the rimstone dams discussed above. All of the no longer active travertine in the Cave is certainly an indication of greater effective moisture in the cave. Unfortunately, none have been dated so the timing (or timings) of this greater effective moisture is not known.

Goat (Capra hircus) bones are common on the surface of the cave floor. Goats commonly seek shelter in the cave and occasionally die in it or on the talus slope. They provide a good analogy for some of the bone in the Hall's Cave deposit. At least five goats died in the cave during the winter of 1988- 1989.

Composite Pit I

Much of the discussion above concerned CompOSite Pit I, its stratigraphy and the animals found in it. Refer to the text above as a guide to what to look for in Composite Pit I.

17

REFERENCES CITED

Barnes, V.E. (project director) (1981) Liano Sheet Geologic Atlas of Texas Austin, TX: Bureau of Economic Geology. 15pp + map.

Blair, W.F. (1950) The biotic provinces of Texas The Texas Journal of Science. 2: 93-117.

Burt, W. H., and Grossenheider, R. P., (1976) A FieJd Guide to the Mammals (3rd editioni: Boston: Houghton Mifflin Co., 289p.

Dalquest, W.W., Roth, E., and Judd, F. (1969) The mammalian fauna of Schulze Cave, Edwards County, Texas. Bul/etin of the Florida State Museum 13: 205-276.

Davis, W.B. (1974) The mammals of Texas (revised). Texas Parks and Wildlife Bulletin 41: 294pp

Davis, W.B., Ramsey, R.R. and Arendale, Jr. J.M. (1938) Distribution of pocket gophers (Geomys breviceps) in relation to soils. Journal of Mammalogy 19: 412-418.

Dittemore, W.H. and Coburn, W.C. (1986) Soil Survey of Kerr County, Texas. U.S.D.A., Soil Conservation Service and Texas Agricultural Experiment Station. 123pp + plates. . -

Dixon, J.R. (1987) Amphibians and Reptiles of Texas with Keys, Taxonomic Synopses, Bibliography, and Distribution Maps College Station, TX: Texas A & M University Press 434pp.

Dodson, P. and Wexlar, D. (1979) Taphonomic investigations of owl pellets. Paleobiology 5: 275-284.

Downhower, J.F. and Hall, E.R. (1966) The pocket gopher in Kansas University of Kansas Museum of Natural History Miscellaneous Publicatrions 44: 1-32.

Graham, R.W. (1976) Pleistocene and Holocene Mammals, Taphonomy, and Paleoecology of the Friesenhahn Cave Local Fauna, Bexar County, Texas unpublished Ph.D. dissertation: University of Texas at Austin.

Graham, R.W. and Lundelius, E.L. (1984) Coevolutionary disequilibrium and Pleistocene extinctions IN (P.S. Martin and R.G. Klein, eds) Quaternary Extinctions Tucson: University of Arizona Press. pp. 223-249.

Hall, E.R. (1981) Mammals of North America (2nd edition). New York: John Wiley and Sons. 2 Vols.

Kastning, E.H. (1983) Geomorphology and Hydrogeology of the Edwards Plateau Karst, Central Texas. Unpublished Ph.D. dissertation UT Austin. 656pp.

Kasting, E.H. (1986) Cavern development in the New Braunfels area, central Texas IN (P.L. Abbott and C.M. Woodruff, jr. eds.) The Balcones Escarpment published for the Geological Society of America Annual Meeting Nov. 1986, San Antonio, Tx. pp 91-100.

Kasting, E.H. (1987) Solution-subsidence-collapse in central Texas: Ordovician to Quaternary IN (B.F. Beck and W.L. Wilson, eds.) Karst Hydrogeology: Engineering and Environmental Applications Rotterdam: A.A. Aalkema . pp 41-45.

Lundelius, Jr. E.L. (1967) Late-Pleistocene and Holocene faunal history of central Texas IN (P.S. Martin and H.E. Wright ,Jr. eds.) Pleistocene Extinctions New Haven: Yale University Press pp.287-319.

Lundelius, Jr. E.L., Graham, R.W., Anderson, E., Guilday, J. Holman, J.A., Steadman, D.W. and Webb, S.D. (1983) Terrestrial vertebrate faunas IN (S.C. Porter, ed.)Late Quaternary Environments of the United States, Vol. 1 The Late Pleistocene Minneapolis: Univ. of Minnesota Press. pp 311-353.

Manning, R.w., Jones, Jr., J.K., and Jones, C. (1989) Comments on distribution and variation in the Big Brown Bat, Eptesicus fuscus, in Texas, Texas Journal of Science 41: 95-101.

1 8

Martin, R.F. (1981) Reproductive correlates of environmental variation and niche expansion in the cave swallows in Texas, Wilson Bulletin 93: 506-518.

McMahan, C.A., Frye, R.G., and Brown, K.L. (1984) The Vegetation Types of Texas Including Cropland, Austin, TX: Texas Parks and Wildlife Department. 40pp and map.

McNab, B.K. (1974) The behavior of temperate cave bats in a subtropical environment Ecology 55: 943-958.

Patton, T.H. (1963) Fossil vertebrates from Miller's Cave Llano County, Texas. Bulletin of the Texas Memorial Museum 7: 1-41.

Rodda, P.U., Fisher, W.L., Payne, W.R., and Schofield, D.A. (1966) Limestone and dolomite resources, Lower Cretaceous Rocks, Texas Bureau of Economic Geology, Report of Investigations 56 286pp.

Rose, P.R. (1972) Edwards Group, surface and subsurtace, central Texas Bureau of Economic Geology, Report of Investigations 74 198pp.

Roth, E.L. (1972) Late Pleistocene mammals from Klein Cave, Kerr County, Texas The Texas Journal of Science 24: 75-84.

Semken, Jr., H.A. (1961) Fossil vertebrates from Longhorn Cavern Bumet County, Texas. The Texas Journal of Science 13: 290- 310.

Texas Department of Parks and Wildlife (1986) Kerr Wildlife Management Area Driving Tour Austin, Tx: Texas Dept. of Parks and Wildlife. 8pp.

Weniger, D. (1988) Vegetation before 1860 IN (B.B. Amos and F.R. Gehlbach, eds.) Edwards Plateau Vegetation Waco, Tx: Baylor University Press. pp 17-23.

Wolfe, L.R. (1965) Checklist of Birds of Kerr County, Texas. Kerrville, TX: published by author. 17pp.

Acknowledgements

The work at Hall's Cave has been graciously supported by the Cave Research Foundataion, The Geology Foundation of the University of Texas at Austin, Geological Society of America, and Sigma Xi. I would also like to thank Dr. Ernest Lundelius, Dr. Stephen A. Hall, Dr. Timothy Rowe and Dr. Robert L. Folk for their continuing help in this project. T.D. Hall has been a most warm and hospitable host, allowing me to work at his cave. Dr. Russell W. Graham and Dr. James G. Mead provided important help and discussions. Leland Bement provided much needed archaeological insight. I would also like to thank Billy Davidson and Vickie King for their invaluable assistance in picking Hail's Cave matrix. I thank Robert S. Rosenberg for suggesting that I work on Hall's Cave and for his help in the field. John Chaille was a valuable field assistant. John Buckley, Sally Shelton and Sally Zellers each provided important editorial assistance. None of this would have been possible without the hard work and assistance of the staff and students of the Vertebrate Paleontology Lab, Texas Memorial Museum.

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