BUREAU OF ECONOMIC GEOLOGY The University of Texas Austin 12, Texas

Peter T. Flawn, Director

Report of Investigations—No. 45

Pleistocene Molluscan Faunas and Physiographic History of Pecos Valley in Texas

By A. BYRON LEONARD AND JOHN C. FRYE

July 1962

BUREAU OF ECONOMIC GEOLOGY The University of Texas Austin

REPORT OF INVESTIGATIONS No. 44 Errata

Page— 263, line 26: "Wilson et al. (1927)" should read "Wilson et al. (1957)"

263, line 27* "Gardner (1953)" should read "Gardner (1933)"

266, line 1: "Ackerman Formation" should be in regular heavy letters

267, last major heading: "Betheden Formation" should be in regular heavy letters

267, last line: "Lithology. 25 feet, locally absent." should read "Bauxitic and kaolinitic clay; lignite."

286, line Is "Squirrel Creek Formation" should read "Squirrelcreek Formation"

Please correct your copy

BUREAU OF ECONOMIC GEOLOGY The University of Texas Austin 12, Texas

Peter T. Flawn, Director

Report of Investigations—No. 45

Pleistocene Molluscan Faunas and Physiographic History of Pecos Valley in Texas

By

A. BYRON LEONARD AND JOHN C. FRYE

July 1962

Contents

PAGE Abstract 5 Introduction 5 Physiography 7 Late Cenozoic physiographic development 10 Fossil molluscan faunas 13

Physiographic history of Pecos Valley in relation to fossil molluscan faunas 14 Annotated check list of fossil molluscan species 17

Age and correlation of fossil molluscan faunas . 27

References .... 29 Index 41

Illustrations

FIGURES— PAGE

1. Map of a part of southwestern Texas showing localities from which fossil mollusks are listed and position of generalized cross-profiles in figure 2 6 2. Generalized and schematic cross-profiles of the Pecos Valley region of Texas 8 3. Profile of Pecos Valley flat along trend line of valley; profiles 10 miles each side of the channel; and profile of High Plains surface to north 9 4. Chart showing fossil molluscan faunas collected from Pecos Valley region

of Texas during 1958 and 1959 .. 13

PLATES— I. Some geologic features of the Pecos River valley in Texas 32 11.-IV. Fossil mollusks from the Pecos Valley region of Texas 34-38

Pleistocene Molluscan Faunas and Physiographic History of Pecos Valley in Texas A. BYRON LEONARD 1 and JOHN C. FRYE2

ABSTRACT Studies of physiographic relations and of luscan fossils were observed in the sedi- fossil molluscan faunas made in the Pecos ments of Surface I; meager collections in River valley region (Val Verde County to alluvial sediments underlying Surface II the Texas—New Mexico border) reveal ex- confirm its Pleistocene(?) age; adequate tensive, well-pedimented surfaces southwest collections of fossil mollusks from alluvial of the river extending from the Davis sediments beneath Surface 111 are charac- Mountains to near the present channel. teristic of early Wisconsinan, and the These surfaces are described as Surface I, uniquely rich faunas in deposits below graded to the late Tertiary Ogallala drain- Surface IV are characteristic of post- ageways; Surface 11, graded to a lower Twocreekan (post-Bradyan) assemblages. level Pleistocene channel of Pecos early Physiographic features of Pecos River val- River and extensively pedimented during ley are illustrated by generalized profiles Illinoian time; Surface 111, graded during and photographs; the 33 localities from early Wisconsinan to a level only slightly which fossil mollusks collected in- above the present channel of Pecos River; were are and Surface IV, developed during latest dicated on a location map, the species in- Wisconsinan and Recent as a terrace of cluded in each local molluscan faunal as- highly fossiliferous deposits in which the semblage are shown on a chart, and each Pecos River is presently incised. No mol- species is illustrated photographically. INTRODUCTION The Pecos River extends generally south- relations had been made from farther north easterly across southwestern Texas from the in western Texas southward into Howard New Mexico line to its junction with the County, thus furnishing a point of depar- Rio Grande on the Mexican border. The ture for southwestward correlations. complex valley through which the river Although fossil snailshave been obtained flows in Texas is an extensive region from from 33 localities within the Pecos Valley which Pleistocene fossil molluscan faunas region (fig. 1) a significant geographic gap are virtually unknown, and from which de- still exists between the faunas previously tailed information on the physiographic reported (Frye and Leonard, 1957) in history and the stratigraphy of the surficial Howard and Martin counties and the most deposits is sparse. During August 1958 and northeasterly localities here reported from June 1959 the writers undertook a recon- Crane, Pecos, and Crockett counties. Fur- naissance of Pleistocene fossil mollusks of thermore, many of the faunas of the Pecos the Pecos Valley region. Snail faunas had Valley region are not clearly definitive as to previously been described (Frye and Leon- age and correlation. ard, 1957) as far south as northern Glass- It is the purpose of this paper to report cock County, and general Pleistocene cor- only upon the Pleistocene fossil mollusks obtained from this and to a 1 Department of Zoology, University of Kansas, Lawrence. region present 2 Chief, Illinois Stale Geological Surrey, Urbana. tentative outline of the physiographic his- 6 Report of Investigations—No. 45

Tory of the Pecos Valley in Texas. A de- and present inadequate knowledge of the tailed discussion of Pleistocene stratig- faunas. The based raphy is not attempted because of the pau- work on which this report is was done under the auspices of the Bureau city of adequate exposures, lack of detailed of Economic Geology of The University of knowledge of physiographic surfaces, lack Texas and is part of a broader project on of clearly recognizable lithologic criteria late Cenozoic geology in western and north- such as volcanic ash beds and buried soils, ern Texas.

Fig. 1. Map of a part of southwestern Texas showing localities from which fossil mollusks are listed and position of generalized cross-profiles in figure 2. PHYSIOGRAPHY

The region of the middle part of the 2,900 feet to less than 1,300 feet at its junc- Pecos Valley is sufficiently distinct in land tion with the Rio Grande. Although the forms and climate that it has long been rec- character of the valley and its local gradient ognized (Fenneman, 1931) as a distinctive change radically from place to place (figs. physiographic section. Upstream from Up- 2 and 3) the channel is near bedrock ton County, Texas, it is a semiarid plains throughout the entire distance across Texas. region, characterized by gently sloping and The striking influence of the local bedrock relatively undissected surfaces, and broken on valley configuration is graphically at places by mesas of various sizes thatrise shown in figures 2 and 3. On figure 2, the sharply above the regional plain. Southwest generalized cross-profiles ( see fig. 1 for of the Pecos River the assemblage of slop- locations) A through D are in an area ing surfaces rises gradually toward the where the bedrock is relatively non- southwest to the flank of the Davis resistant Triassic and Permian, whereas Mountains. the topography shown in generalized cross- Even though it is a semiarid plains re- profiles E through H is controlled by more gion, the character of the topography con- resistant limestones of Cretaceous age. trasts markedly with that of the adjacent The same relationship of topography to High Plains surface and with the broad bedrock control is shown in figure 3 by the flat uplands of the Edwards Plateau (PI. I). generalized profiles drawn parallel to the This contrast in topography coincides with trend of the Pecos River and approximately a contrast in geology. The High Plains are 10miles on either side of the channel. Here, regionally underlain by, and topographi- these flanking profiles roughly simulate the cally reflect the character of, the late Terti- gradient of the channel through the belt ary Ogallala formation, and the upland sur- underlain by Triassic and Permian rocks faces of the Edwards Plateau are all under- but radically depart from it in the area lain by resistant limestones of Cretaceous where the belt of Cretaceous rocks crosses age. In contrast, the Pecos Valley, north- the valley. westward from Crane County, is underlain The effect of local bedrock on channel by Triassic and Permian rocks with scat- gradient is also shown in figure 3. In the tered outliers of more resistant Cretaceous 50-mile segment nearest the mouth of the rocks (Sellards, Adkins, and Plummer, river, which is entirely within Cretaceous 1933). For the most part these more rap- rocks, the gradient of the valley flat is ap- idly erodible Triassic and Permian rocks proximately 12 feet per mile, whereas for are covered by a relatively thin veneer of the next 150miles upstream the gradient of Pleistocene, and locally late Tertiary, the valley flat is less than 5 feet per mile. It deposits. is significant that within the first 50 miles The physiography adjacent to the lower after the valley has entered Cretaceous Pecos River contrasts strongly with that limestone, the gradient does not appreci- adjacent to the middle segment of the val- ably exceed 5 feet to the mile. ley. Along the lower course of the Pecos The strong contrast in valley morphol- River the main valley is a much less con- ogy associated with bedrock stratigraphy spicuous element of the regional topog- can perhaps be best expressed by descrip- raphy. It is a relatively narrow portion of tions of the valley slopes. In the belt of Tri- the drainage basin and displays more assic and Permian outcrop the valley is characters in common with the basin of strongly asymmetrical. A distinct valley and north of the Devils River, to the east, than it does with wall (PI. I, A F) occurs its own basin farther upstream in Texas. river from Loving County across Ward The channel of the Pecos River across County and part of Crane County. This Texas descends from an elevation of nearly north valley wall generally is little more 8 Report of Investigations—No. 45

FIG. 2. Generalized and schematic cross-profiles of the Pecos Valley region of Texas. Based on 100- foot contour interval, 1:250,000-scale maps of Pecos, San Angelo, Fort Stockton, and Sonora areas issued by U. S. Geological Survey.

than 100 feet in height, but throughout a presents a consistent contrast with the significant part of the distance Triassic north valley side slope. In eastern Reeves rocks are exposed below capping Cenozoic County and northwestern and north-cen- deposits. The valley wall generally lies from tral Pecos County, pediments of regional 1 to 5 miles from the Pecos River channel. proportion extend southward from the In this area the south valley side slope Pecos channel bank (PI. I, D; fig. 2) and Pleistocene Molluscan Faunas, Pecos Valley, Texas 9 in some areas extend as an almost un- The strong asymmetry of the valley and broken sloping surface for more than 30 the extensive pediment surfaces are both miles to the south and southwest to a point abruptly reduced where Pecos River enters where they approach the flank of the Davis the outcrop of Cretaceous limestones west Mountains. Although locally interrupted by of McCamey. Progressively narrower pedi- mesas of Cretaceous limestone (PL I, E), ments persist for more than 50 miles down- the pediment surfaces generally rise to ele- stream from McCamey, but it is the gravity vations of more than 1,000 feet above the slope and free-face slope elements that channel of Pecos River. The extreme is dominate the topography. Through this represented by a complex of integrated stretch the valley, even though sharply con- pediment surfaces that may be traced con- stricted near McCamey, progressively nar- tinuously southwestward from the Pecos rows downstream (fig. 2, E, F, G). River channel into northern Brewster Downstream from southwestern Crockett County where pediment surfaces stand at County the valley becomes a sharply in- elevations more than 1,500feet above chan- cised trench (fig. 2, H) lacking all but the nel level. most rudimentary flanking pediments (PL In this area the asymmetry of the valley I, B). In this segment of greatly steepened is accented by the northeastward exten- gradient (fig. 3) the valley floor is domi- sion, from the low crest of the north valley nated by alluvial fans, built at and below wall, of an almost unbroken upland surface the mouths of entering tributaries, that that is indistinguishable from the north- have been dissected and partly cut away ward adjacent parts of the High Plains. by the main stream. The partial destruction This lack of a northward physiographic of the medium- to coarse-textured fan de- boundary contrasts with the imposing posits has produced locally prominent un- mountain fronts that serve as a southwest- paired terraces. At other places the stream ern terminus to the more steeply rising flows directly on bedrock and is flanked by pediment surfaces to the south. an insignificant strip of alluvium. The

FIG. 3. Profile of Pecos Valley flat along trend line of valley; profiles 10 miles each side of the channel; and profile of High Plains surface to north. Based on 100-foot contour interval, 1:250,000- scale maps of Pecos, San Angelo, Fort Stockton, and Sonora areas issued by U. S. Geological Survey. 10 Report of Investigations—No. 45 absence of flanking pfediments coincides Plains surface shifts to a southerly direc- with the segment of greatly steepened val- tion. This transition from an east-southeast ley gradient. slope to a southerly slope takes place in an indistinctly definable belt along a north- LATE CENOZOIC PHYSIOGRAPHIC west-southeast-trending zone across Ector DEVELOPMENT County where Cretaceous rocks are at or In spite of the many uncertainties in the near the surface. This vaguely defined di- interpretation of late Cenozoic history of vide continues into northwestern Upton the Pecos Valley in Texas, it nevertheless County and appears to be continuous with seems desirable to present an outline of its the high Cretaceous bedrock including inferred history as background to under- King Mountain near McCamey. Because standing the Pleistocene fossil molluscan of inadequate exposures, it has not been faunas to be described. Northward from possible to determine whether or not this this part of Texas extends the vast plateau divide zone consistently lacks Ogallala de- surface called the High Plains. The surface posits, but it is apparent that at least in of this plateau is generally underlain by Ector and Winkler counties the physio- sediments of the late Tertiary Ogallala graphic continuity of the High Plains sur- formation. This formation ranges in thick- face is uninterrupted across it. ness from a few feet to more than 500 feet This shift in High Plains surface slope and has been correlated into the panhandle strongly suggests a southward drainage region of Texas and northward across Kan- trend of the late Tertiary alluviating sas and Nebraska (Frye and Leonard, streams along the north side of the middle 1957, 1959). It reached its culmination of Pecos Valley and a drainage outlet to the sedimentation by mid-to late-Pliocene time. southeast in the general position of the Although locally thin Ogallala formation lower Pecos River basin. The continuity persists to the crest of the north wall of the of the surface and lack of locally distin- Pecos Valley (PI. I, A, F), it is generally guishable expression of a divide, together thin, discontinuous, and patchy southwest with the possibility of discontinuous from Howard County. The continuous ex- Ogallala deposits across the area, indicate tent of the topographic surface, or physio- the integrated nature of the High Plains graphic datum defined by the High Plains, erosional-depositional surface and suggest however, can be traced in Midland, Ector, that the Ector County late Tertiary divide Winkler, Ward, Crane, and Upton coun- was of no greater significance than other ties, where it is in part a constructional divide areas that have been recognized oc- and in part an erosional surface. In the curring on the High Plains surface to the southwestern part of this area the continu- north. ation of this late Tertiary surface is exten- Granting the continuity of the High sively veneered with relatively young Plains surface across Ector County, the Pleistocene deposits, but in southern How- late Tertiary coalescent alluvial plain ard, Glasscock, and Upton counties (fig. clearly was terminated on the southwest 2, E, F) large mesas and buttes capped by by the toe of the extensive pediments rising resistant Cretaceous limestones stand sev- to the flanks of the Davis Mountains. Thus, eral hundred feet above it. It is evident in late Tertiary time the highest pediment that this late Tertiary coalescent alluvial surfaces to the southwest were in a flank- plain, that engulfed all former topography ing position to the southernmost through- for hundreds of miles to the north, became, flowing drainage line of the High Plains in this area, a lowland plain below the rem- complex. However, southeastward from nants of a higher surface that had been eastern Crane County this was not the case cut earlier on the Cretaceous limestones. as the Pecos Valley was no longer inte- In Crane, Ector, Ward, and Winkler grated with the High Plains complex but counties the gradient or slope of the High was isolated from the plains by the essenti- Pleistocene Molluscan Faunas, Pecos Valley, Texas 11 ally continuous higher bedrock Cretaceous The sequence of pediment surfaces is upland (figs. 2 and 3). That an ancestral difficult to determine because of the many late Tertiary Pecos River existed across the local control levels. A suggested sequence Cretaceous bedrock area nevertheless is in- has been arrived at for the area southwest dicated by (1) the suggested correlation of from the city of Pecos, and these surfaces the highest pediment level containing well- tentatively have been correlated through- developed pisolitic limestone on its surface out the region (fig. 2). For ease of discus- into the narrowed Pecos Valley south of sion the recognized surfaces have been McCamey, by (2) the requirement of a numbered by Roman numerals from high- southeasterly drainage outlet as control for est to lowest. the extensively developed pediments as Surface I was observed only in relatively well as an outlet for the late Tertiary small areas and generally adjacent to pedi- alluviating streams across Crane, Ward, ment heads. It is our judgment that Surface and Winkler counties, and by (3) scattered I is the late Tertiary surface, contempo- remnants of alluvial deposits on relatively raneous in its climax development with high bedrock at a few places in Val Verde the High Plains to the northeast and the County. The position of the late Tertiary remnants of Ogallala, in which a few frag- Pecos River southeastward from eastern mentary Biorbia seeds have been observed, Pecos County may have departed markedly in northwestern Pecos County. If our cor- from that of the present stream and it relation of this surface into eastern Pecos seems probable that much of the entrench- County is correct (fig. 2, F), it indicates ment of the drainage of northeastern Ter- that the narrow late Tertiary pediment rell, southwestern Crockett, and western along the eastern part of the Pecos Valley Val Verde counties took place during the was significantly lower than the High Pleistocene. Plains surface to the north of the Creta- The Pleistocene history of the northern ceous divide. This suggests a steeper gradi- half of the Pecos Valley in Texas, although ent on the late Tertiary streams along the obscure in detail, seems evident in its Pecos Valley than obtained along the major aspects. It is probable that in general High Plains streams farther north in this segment of the channel has been low- Texas. ered no more than 300 feet below its late Surface II constitutes the most extensive Tertiary level. It is evident that the ex- surface of Reeves and western Pecos tensive region including much of Pecos counties. It is veneered by several genera- and Reeves counties has existed as a com- tions of deposits that suggest several cycles plex of pediment surfaces at least since of activity. The sediments range in compo- mid-Tertiary time and that the control of sition from coarse-textured, cross-bedded these pediments has been by through-flow- stream gravels that are judged to be the ing drainage in a position relatively near result of streams that crossed the pediment the present position of Pecos River. Pedi- surfaces, to unoriented, poorly sorted, and ment veneers have consisted of coarse locally caliche-cemented colluvium. The colluvium interspersed with strips of allu- position of this surface, distinctly below vium along streams flowing generally to- Surface I, the presence of sparse snail ward the northeast and serving as local shells ( Succinea ) in the youngest of the control for their systems of flanking own multiple generations of pediment veneer, pediments. Significant areas of uncon- and its position above more richly fossil- sumed upland exist as mesas, or inselbergs, iferous deposits lead to the conclusion that in the eastern part of this region of pedi- Pleistocene in ment complex, and some small areas of it is pre-Wisconsinan age. Ogallala formation are thought to be pres- Its development may have taken place dur- ent in northwestern Pecos County and ad- ing the first half to two-thirds of Pleisto- jacent Reeves County. cene time, and may have been controlled 12 Report of Investigations—No. 45 by a relatively stable stream position that 111 is not firmly established. Moa was maintained by the resistant rocks is early Wisconsinan, although b downstream. One or more of the broad sags sible that it may be older wr that cross Ward and Crane counties, enter- Pleistocene. ing the Pecos Valley from the north, may Surface IV is of relatively m have been produced by southward to south- tent and consists mostly of the fir southwestward-flowing streams during the of Pecos River and its major ttf® more humid early Pleistocene (Frye and (PI. I, A). In some places Su* Leonard, 1957a) and thus be contempo- includes narrow flanking pedime raneous with this extensive pedimented are continuous with the flood-plain surface south of the Pecos River. and are distinguished from Surfao Surface 111 is generally distinguished a change in the angle of slope. T. from Surface II only by a change in the molluscan faunas reported from ti angle of slope, but locally a low pediment were collected from channel-baa wall has been observed separating the two sures of the alluvial deposits. The' surfaces. In areal extent it is only a small deposits, particularly downstreaj fraction of the size of Surface 11. At many central Ward County, constitute tw places its pediment toe is accordant with of fill separated by a distinct but ri the alluvial deposits of tributary streams immature soil. The uppermost cyi and of the Although a few fossil Pecos. have been periodically receiving snail shells have been found the pedi- at in from overbank flooding during ment veneers of this surface, most of the times. An age of late Wisconsinan ; faunas attributed to this level were col- cent is indicated for the deposits bel lected from the associated alluvial deposits face IV because of (1) their activ (PI. I, C) that were formed by streams plain situation, (2) their confon flowing down the pediment slope. These existing stream gradients, and (3) tl streams controlled the development of their molluscan faunas they have yielded. local flanking pediments, and they in turn were controlled by the position of Pecos In general the Pleistocene hisl Pecos River. It is possible that one or more of Valley from Loving to C the sags entering Pecos Valley from the counties has been characterized by north across Ward and Crane counties sive pedimentation, whereas from C were contemporaneous with Surface 111 County to the mouth of the river rather than with a late phase of the devel- Verde County it has been character opment of Surface 11. The age of Surface sharp valley incision. FOSSIL MOLLUSCAN FAUNAS ssil molluscan faunas discussed in from the northwestern corner of Val Verde (y were collected by Frye and County to near the southern border of New in the summer of 1958 and in the Mexico. ; summer by Leonard and Tong- In local exposures where fossil mollusks who concentrated their attention were sparse, collections were made by [iferous deposits exposed in the hand-picking, but Leonard and Ho em- banks of Pecos River. The 33 lo- ployed the wet-screening method exten- ;rom which fossils were collected sively in making their collections from the ributed from Val Verde County channel bank exposures in Pecos River. f 32) to northern Reeves County All fossil mollusks discussed in this report f 10) near the Texas-New are housed in the collections of late border. As indicated earlier, fossil Cenozoic and Recent Mollusca at The Uni- g localities are indeed rare in the versity of Kansas (all catalogue numbers leistocene sediments that consist refer to these collections), except repre- if alluvial deposits now beveled by, sentatives deposited with The University of ied beneath, an almost unbroken Texas Bureau of Economic Geology, if pediment slope colluvium. More Austin, Texas. A grant from the National rous exposures and more abundant Science Foundation (NSF-G3481) to the in faunas were found in deposits senior author also assisted in the support to be Wisconsinan in age; in fact, of the study. The writers take this oppor- {abundant and populous exposures tunity to acknowledge the able assistance ipost-Twocreekan terrace deposits, of Mr. Tong-yun Ho, graduate student at b Pecos River is presently incised, The University ofKansas.

Pecos Valley region of Texas during 1. Chart showing fossil molluscan faunas collected from d 1959. 14 Report of Investigations—No. 45

PHYSIOGRAPHIC HISTORY OF PECOS VALLEY IN RELATION TO FOSSIL MOLLUSCAN FAUNAS

Several features of the physiographic been made (unfortunately, from the view development of Pecos River valley deserve of the stratigraphic paleontologist) in further comment here, inasmuch as they search of gravels for construction and road have profoundly affected in the past both materials, thus failing to expose the finer the nature and numbers of molluscan popu- alluvial sediments that must have been de- lations as well as the preservation of mollus- posited by streams flowing out of the Davis can shells as fossils. Alluvial deposits and Mountains. On the northeastern flank of the pediment slope veneers that cover them, Pecos Valley, sediments approximately underlying Surface I and Surface II (PL equivalent in age to the stream deposits I, F; fig. 2), are not generally exposed, but under Surface I consist of graveliferous at a few places pits and roadside excava- Ogallala formation, from which a few frag- tions are sufficient to develop a generalized mentary seeds, but no fossil mollusks, have picture of the nature of the sediments been recovered. Under these circumstances, beneath these surfaces. it is not now possible to develop any Surface I, judged to be late Tertiary in knowledge of molluscan faunas related to its primary development, flanks the Davis the pre-pedimented phase of development Mountains to the northeast; exposures of Surface I, since the depositional en- relatively near the mountains, observed vironment of channel gravels is generally along U. S. Highway 67 northeast of Al- inimical to the preservation of fossil pine, Texas, show the presently well- mollusks. pedimented and essentially unbroken sur- Only meager exposures of the alluvial face to be underlain by extremely coarse sediments underlying Surface II were ob- elastics, consisting of boulders up to several served, but these were sufficient to indicate feet in diameter, cobbles, gravel and sand, the presence of a former shallowly stream- with only minor increments of fine sedi- dissected topography that was graded to an ments. At other localities, however, the early Pleistocene ancestral Pecos River at a sediments underlying this surface exposed somewhat lower level than that existing in in a few gravel pits and roadside excava- late Tertiary time. It is judged that Surface tions consist of cobbles up to several inches II was initiated by stream dissection of Sur- in diameter, gravel and sand, with lentils face I in response to a lowered local base and minor strata of sandy silt. Admitting level and increased precipitation, followed the paucity of exposures of the alluvial and by smoothing by pedimentation in re- colluvial materials underlying Surface I, it sponse to the same local base level but de- is nevertheless possible to deduce that the creased precipitation. Although the average ancestral Surface I presented a shallow- particle size of the elastics is noticeably re- incised coarse-textured topography, graded duced, consisting of cobbles of igneous to the southernmost through-flowing Ogal- rocks from the Davis Mountains, reworked lala drainageway, and contemporaneous caliche from Surface I materials, gravel, in development with the well- locally sand, and increased amounts of fine sand preserved Ogallala formation (PI. I, A, D) and silt, the depositional environment was on the eastern flank of the valley of Pecos not favorable to the of River. Episodes of pedimentation during preservation fossil mollusks. At a the latest Tertiary and again during the few places, such as Locality latter half of the Pleistocene have beveled 33, a few gastropods of a kind not known and veneered Surface I so perfectly that to occur in late Tertiary faunas ( Succinea) exposures of the underlying sediments are were helpful in identifying this as the oldest almost completely limited to those pro- of the Pleistocene surfaces. Again, lacking duced by the activities of man. These have all but graveliferous exposures, no concept Pleistocene Molluscan Faunas, Pecos Valley, Texas 15 of the contemporary molluscan faunas as- in nearby areas, that the cover sands were sociated with the integrated stream pattern spread as a generally uniform sheet by ac- across the pre-pedimented Surface II to- tivity of the wind acting on a surface pography is yet possible, but deposits in scantily provided with scattered clumps of Crane County (Localities 1, 2) are judged vegetation. The important consideration to be at least approximately equivalent in here is the evidence of arid conditions in age to alluvial deposits underlying Surface Illinoian time in this part of Texas. The 11. There is no way at present of ascertain- writers propose that the initial shift from ing, however whether or not the molluscan an integrated stream pattern on Surface II fossils reported from the Crane County lo- began with the trend toward arid climatic calities give much insight into the nature of conditions in western Texas in Illinoian contemporary faunas associated with Sur- time; it is obvious, of course, that pedimen- face II on the southwestern flank of the tation has generally continued into Recent Pecos Valley, since local environmental time, although there may well have been conditions there may well have been sig- minor fluctuations toward an integrated nificantly different. stream pattern on these surfaces during As already noted, Surface I and Surface pluvial intervals. The pediment veneers re- II are now well pedimented, a surficial ve- flect a trend toward the present-day arid neer of colluvial materials having beveled climate in the Trans-Pecos region, inas- across the ancient shallowly incised coarse- much as the deeper parts of the veneers textured topography. The surficial aspect of contain, at least locally, extremely coarse Surface I and Surface II is now generally elastics, while surficial materials under unbroken and unrelieved, although active transit today scarcely ever exceed the size transport of relatively fine sediments still of medium to fine sand, and the predomi- continues in this almost desert climate nant clastic is silt. The gradual and con- under a scant cover of vegetation. tinuous reduction in particle size of the Although the time of the initiation of the pediment veneers, from the deeper to sur- colluvial blanket now veneering Surfaces I ficial materials, reflects the general cli- and II is difficult to determine with accu- matic deterioration in this region, with racy, it is judged that on Surface II it dates consequent reduction in competency of run- from the smoothing of the stream-textured off waters to move sediments down slope, topography caused by the trend toward since changes in gradient seem not to be greater aridity that characterizes the latter sufficiently significant to account for ob- half of the Pleistocene. Unfortunately, there served differences in grain size. is no molluscan faunal control, since collu- Best observed exposures of Surface 111 viated elastics, especially those containing sediments, rather than being under the the cobbles, gravels, and sands character- pediment surface itself, consisted of alluvial izing the deeper parts of these veneers, are deposits equivalent to Surface 111, repre- scarcely conducive to the preservation of sented by segments of stream deposits in- molluscan shells. Moreover, there is reason cised below the level of Surface 11. Repre- to judge on other grounds that climatic de- sentative of these deposits are those shown terioration contemporary with the initia- on Plate I, C; nearby at Toyah, Reeves tion of pedimentation may have seriously County, a rather abundant and varied fos- reduced molluscan populations. Frye and sil molluscan fauna (fig. 4, Locality 13), Leonard (1957, p. 28) showed that the judged to be early Wisconsinan, was ob- broad expanses of "cover sand" on the tained. The sediments in question indicate High Plains of northwestern Texas are Illi- that an environment conducive to mollus- noian in age and eolian in origin. They can life existed along the stream courses deduced, from the nature of the deposit and that crossed Surface 111. In the deposits the observations on eolian movement of sands preponderant clastic is grayish-tan to rusty- 16 Report of Investigations—No. 45 tan silt, containing some fine to medium nel banks of Pecos River expose above low- sand and small increments of clay, and hav- water stage 20 to 25 feet of the sediments ing in places thin lenticular beds of gravel. of Surface IV, although locally (PI. I, A) The presence in the contained faunas of the channel is cut below these sediments unionid and sphaeriid pelecypods, as well into Triassic bedrock. Except for 2 to 4 feet as series of aquatic pulmonate gastropods, of Recent overwash consisting of fine to me- indicates that relatively permanent streams dium sand and very little silt, the channel characterized the topography of Surface bank deposits are involved in a weakly de- 111, while abundant terrestrial gastropods veloped but distinctly zonal soil. In such an attest an adjacent vegetative cover signifi- exposure studied in detail in the right bank cantly better than that existing locally of the Pecos River above the mouth of today. Independence Creek on the Chandler ranch If judgments that Pleistocene pedimenta- in northeastern Terrell County (Locality tion of the southwestern slopes of the Pecos 30), in the left bank of Pecos River, 1 mile Valley began in Illinoian time are sound east of the city of Pecos in Ward County and if the writers' judgments that mollus- (Locality 7), and elsewhere, the A-horizon can faunas recovered from deposits associ- is a black, sandy, friable silt, 18 inches to ated with Surface 111 are early Wiscon- 2 feet in thickness. It seems never to have sinan in age are reasonable, then the pres- been completely leached of carbonates as ence of stream deposits such as those dis- it contains mollusk shells, but it has in ad- cussed above, points to a revival of an inte- dition been enriched by carbonate from grated, though quite coarse-textured, the overwash sediments above. The B- stream pattern in the area during early horizon, although displaying little textural Wisconsinan time. It is entirely possible, if contrast, ranges locally from 18 inches to not probable, that climatic deterioration 21/ 2 feet in thickness; the sediments are and consequent trend toward pedimenta- red brown to reddish tan, have a typical tion did not reach its peak until post-Two- prismatic structure (although clay enrich- creekan time, and that the unrelieved pedi- ment is slight), and contain fossil shells. ment surfaces forming the conspicuous The zone of lime enrichment, varying from topographic feature of the southwestern yellow tan to grayish tan, is abundantly Pecos Valley did not reach their final form fossiliferous, and is profusely infiltrated until very late Pleistocene time. As has with diffused carbonate or with soft caliche been stated earlier, lacking faunal control, nodules. The thickness exposed ranges up the segments of time represented in the to 18 feet, but since it was observed that the pediment veneers remain unknown except zone of lime accumulation locally ex- in broad generalities. tended several feet below low-water level, the exact The sediments containing a faunal lo- maximum thickness has not been cality in Ward County (Locality 6) repre- determined. The nature sent, in the writers' judgment, the equiva- of the molluscan faunas and their population lent of Surface 111 on the northeastern large numbers, together with flank of the Pecos Valley. the deeply humic-stained sediments By far the most generally and abun- immediately underlying Surface IV, indi- cate that during dantly fossiliferous of the sediments studied the deposition of this late Pleistocene alluvium the Pecos River was were those below Surface IV, which is de- essentially a slack-water stream flanked by veloped as a late Pleistocene (post-Two- back swamp marshes from central Reeves creekan) terrace, often a mile or more in County to northeastern Terrell County. and width, exposed in the channel banks Elsewhere in western Texas and across the of Pecos River. From near the northwest High Plains in general, all available evi- corner of Val Verde County to well above dence points to a trend toward aridity fol- the city of Pecos, Reeves County, the chan- lowing Twocreekan time. One of the fac- Pleistocene Molluscan Faunas, Pecos Valley, Texas 17 tors that contributed to the choking of the vation of formerly existing fossil molluscan Pecos River, and the production of a local faunas is the apparent shift of the channel environment highly favorable to mollusks of the ancestral Pecos River toward the in the near vicinity of the channel, was the northeast. Almost nothing remains of any decline in competency of the stream with but late Pleistocene terrace materials on the well-documented increasing aridity. the northeastern flank of the valley. However, at least as important as regional Finally, the unusual ecological and climatic control would seem to be that im- paleontological features of the post-Two- posed by variations in resistance to erosion creekan Pecos River deserve comment, of individual Cretaceous bedrock strata at Whatever the complex of factors that re- or near the nickpoint in the deep entrench- duced the competency of Pecos River dur- ment of Pecos River in central Val Verde ing this segment of time, the result was the County. This sensitivity of Pecos River to production of a narrow band of extremely control by the regimen of the lower reaches favorable habitat for mollusks at a time of the river is indicated by the fact that the when regional ecological conditions were lime-accumulation zone of Surface IV sedi- badly deteriorated. Surface IV sediments ments has in the recent past been more along the Pecos River are unique in the deeply incised than at present. Further Great Plains for their rich and varied evidence of this control is seen in the fact molluscan faunas. There is evidence, based, that Surface II represents a complex of however, on a single fossiliferous outcrop pediment surfaces that cannot be corre- (Locality 6), that ecological conditions in lated with individual glacial stages, al- the near vicinity of the Pecos were favor- though there is good reason to judge that able to mollusks in early Wisconsinan physical phenomena associated with both time, but this is not at all unusual, since the Nebraskan and the Kansan stage, and rich early Wisconsinan faunas are the rule possibly part of the Illinoian stage of the rather than the exception, not only in Pleistocene, are represented in the develop- Texas, but elsewhere. ment of this surface. In summary, pediment veneers have de- ANNOTATED CHECK LIST OF FOSSIL stroyed or buried all but extremely scant MOLLUSCAN SPECIES evidence of pre-Wisconsinan molluscan faunas in the Pecos Valley region. Since Class LAMELLIBRANCHIATA Surface I has evidently been subjected to Order PRIONODESMACEA at least two cycles of pedimentation, little Family UNIONIDAE fd'Orbigny) hope remains of recovering significant fossil molluscan faunas from beneath this Subfamily LAMPSILINAE surface. The sediments under Surface 11, Genus ACTIONONAIS Fisher and Crosse also almost completely obscured by a pedi- ACT ION" ON AIS sp. ment veneer, offer almost no exposures ex- PL IV, fig. 14 cept those produced by the work of man, and these, for reasons previously eluci- At Locality 13, at the east edge of the dated, are not likely sites in which to find village of Toyah, Reeves County, in sedi- fossil mollusks, and in fact only extremely ments exposed in the banks of a generally rare examples have been found. Fairly ade- dry gully, numerous but poorly preserved quate faunal collections have been made shells of an unidentified species of Actiono- from deposits correlatable with Surface 111, nais were found associated with Pisidium and, as noted, sediments beneath Surface compressum, Sphaerium striatinum, sev- IV are richly fossiliferous. eral species of aquatic pulmonate gastro- Still another aspect of the physiographic pods, and an assemblage of terrestrial development of Pecos River valley that has gastropods. The assemblage is characteris- significantly affected at least the preser- tic of a stream, although the fine texture 18 Report of Investigations—No. 45 of the sediments suggests one without 7, 9, 10, 21, 28, 30, and 31, that is to say, strong currents. The deposit is judged to over much of the area under considera- be early Wisconsinan. tion. So far as known, these are all stations in Wisconsinan deposits; elsewhere Genus LIGUMIA Swainson (Taylor, 1960) P. casertanum has been LIGUMIA sp. reported from deposits ranging in age PL IV, fig. 12 from early Pliocene to Recent. The species A rich population of a species of Li- is tolerant of many habitats, including gumia was found in creek bank exposures most aquatic situations except deep water. at Locality 6, Ward County. The shells are It is widely distributed over the world, poorly preserved, but it is likely that they and in the Western Hemisphere from represent some form of L. recta. Associ- Alaska to southern South America. The ated with them was another, unidentified shells respond to local conditions, tending and unionid mussel (PI. IV, fig. 13) to become heavier in stream habitats and several species of aquatic gastropods as more delicate in pond situations. A well as an assemblage of terrestrial snails. majority of the shells obtained from Sur- No unionid mussels were found in deposits face IV deposits are notably thin-shelled, judged to be older than early Wisconsinan. especially those near the top of the ter- The Toyah locality (no. 13) is almost race, supporting the judgment that Pecos certainly early Wisconsinan; Locality 6 River was a sluggish stream at the con- may possibly be somewhat younger but clusion of the building of Surface IV. probably not significantly so.

PISIDIUM COMPRESSUM PRIME Order TELEODESMACEA PL IV, figs. 17, 18 Family SPHAERIDAE Genus SPHAERIUM Scopoli Pisidium compressum is a small clam, SPHAERIUM STRIATINUM (Lamarck) with a characteristic trigonal shell. It is PL IV, figs. 15, 16 distributed in deposits ranging in age from This small sphaeriid clam was found at early Pleistocene to Recent and has been Localities 10, 12, and 13. It was nowhere reported as a fossil by Leonard (1950, abundant, but most numerous specimens fig. 4) and by Taylor (1960, p. 48) from were found at Locality 10. The living localities ranging from western lowa to species is widely distributed in North southern Kansas. Living populations pre- America, from northern Canada to Central fer running water, but the species may be America. Living populations are found in found in well-aerated ponds. In the two a variety of habitats, including large and localities (nos. 13, 24) reported here, P. small streams, as well as lakes and ponds. compressum was not abundant but was The two localities near Toyah, Reeves more numerous at Locality 13, at Toyah, County, indicate a sluggish stream, while Reeves County. the sediments at Locality 10 near Red Bluff, northern Reeves County, are rela- Class GASTROPODA tively coarse and strongly cross-bedded, Subclass PROSOBRANCHIA indicating vigorous stream action. Some Order ARCHAEOGASTROPODA of the specimens listed by Frye and Leon- Family HYDROBIIDAE ard (1957, fig. 9) as Sphaerium sp. from Genus CALIPYRGULA Pilsbry localities in northwestern Texas may be CALIPYRGULA PECOSENSIS Leonard and Ho referred to striatinum. PL 11, fig. 6 Calipyrgula pecosensis Leonard and Genus PISIDIUM Pfeiffer Ho, 1960, vol. p. pi. figs. PISIDIUM CASERTANUM (Poll) Nautilus, 73, 125, 12, 1-3. Pl. IV, figs. 20 19, The type locality of this minute branchi- This small clam was found at Localities ate gastropod is in Surface IV sediments Pleistocene Molluscan Faunas, Pecos Valley, Texas 19 exposed in channel bank of Pecos River, This small branchiate gastropod was 3.5 miles northeast of Imperial, in Crane found at two localities (nos. 28 and 30) County (PI. I, A; fig. 4, Locality 4). This both below Surface IV sediments in the snail is known from many localities in channel banks of Pecos River in Terrell Surface IV exposures in the channel banks County, where it was associated with of Pecos River from Sheffield, Terrell Calipyrgula circumstriata and other County, to central Reeves County. It is species. However, at Locality 29, where almost everywhere abundant in this C. circumstriata was abundant, no Coch- stretch of Pecos River, in late Pleistocene liopa riograndensis occurred. deposits, but has not been found living. It C. riograndensis was found living in seems to have had a habitat preference Independence Creek, both on the Chandler for slack-water streams, or ponds and ox- ranch near the mouth of the stream and bow lakes; below Sheffield it is abruptly also farther upstream, where it was com- replaced in Surface IV sediments by Cali- mon under stones and pieces of driftwood pyrgula circumstriata that probably had in the quieter parts of this cool, fast-flow- different habitat requirements. ing creek. This constitutes a northern ex- tension of the range of this species, it CALIPYRGULA CIRCUMSTRIATA Leonard and Ho having been previously reported from PI. 11, fig. 5 southern Val Verde County near the Rio Calipyrgula circumstriata Leonard and Ho, 1960, Grande. Nautilus, vol. 73, p. 125, pi. 12, figs. 1-3. Family HELICINIDAE The type locality of this minute hydro- Genus HELJCINA Lamarck biid gastropod is in the Surface IV deposits HELICINA ORBICULATA TROPICA Pfeiffer situated a quarter of a mile above the mouth of Independence Creek on the PI. 111, figs. 13-15 Chandler ranch, Terrell County, Texas The figured specimen of this species (Locality 30). Shells were also abundant was not collected in the Pecos River valley in a similar exposure on the W. C. Dunlap but in late Pleistocene deposits in Kinney ranch, 12miles south-southeast of Sheffield, County, west of Brackettville. It may well Terrell County, and a few shells were occur either living or as a fossil in the found in a Surface IV exposure about 3 Pecos Valley region, but the writers did miles east of Sheffield, Terrell County. The not find it there. abrupt replacement of Calipyrgula pecos- Subclass PULMONATA ensis by C. circumstriata in this stretch of Pecos River suggests that the latter was Order BASOMMATOPHORA an inhabitant of clear, fast water, espe- Family LYMNAEIDAE cially since it is associated in the localities The family Lymnaeidae comprises a of occurrence with fossil Cochliopa rio- large group of aquatic pulmonate gastro- grandensis that also lives in good popula- pods of such variable nature that attempts tions in Independence Creek. Careful at a systematic arrangement of the several search, however, failed to reveal living kinds has given rise to much confusion examples of C. circumstriata in Independ- and controversy. Extremes of treatment from of nearly a dozen ence Creek or elsewhere. range recognition genera and well over 100 species in North

Genus COCHLIOPA Stimson America to the recognition of a single no more The COCHLIOPA RIOGRANDENSIS Pilsbry and Ferriss genus and than seven species. biological verity probably lies somewhere PI. 11, figs. 7-9 between these extremes. However, inas- Cochliopa riograndensis Pilsbry and Ferriss, much as many of the apparently important Leonard and Ho, 1960, Nautilus, vol. 73, p. 127, pi. 12, figs. 4-7, text figs. 1-4. taxonomic features, especially those sepa- 20 Report of Investigations—No. 45 rating the many named genera, are to be abundant at Localit\ 26 near Fort Stock- found in the soft anatomy, it is deemed ton. Pecos County, where it occurred with advisable to take a conservative attitude less abundant L. parva. Like other species toward fossil shells. Therefore, all kinds of Lymnaea found as fossils in Pecos reported here are assigned to the genus Valley deposits, L. caperata is tolerant of Lymnaea; the shells are so variable that a variety of habitat conditions. Distributed there is sufficient hazard in assigning fossil in a broad belt across North America, it shells to named species. thrives in small, often ephemeral pools, and is capable of withstanding the lack of LYMNAEA BLLIMOIDES BLLIMOIDES (Lea) open waterfor months. PL IV, fig. 4 Lymnaea bulimoides as one form or LYMNAEA PARVA Lea another ranges from middle Pleistocene PL IV, fig. 3 to Recent. Baker (1911, p. 211) indicates A few snails of this species were found that bulimoides east of the Rocky Moun- in the deposits at Locality 26. west of Fort tains are referable either to L. b. cockerelli Stockton. Pecos County. This small gastro- or L. b. the Pecos River valley techella; pod lives in damp situations near open specimens, except those at Locality 6, seem water, rather than in it as do most species best to conform to the typical species. The of Lymnaea. L. parva lives today in popu- characters of the inner lip, the more slender lations distributed from James Bay to last whorl, and longer spire seem to ex- southwestern United States. As a fossil it clude these shells from L. b. cockerelli. exists from lower Pleistocene to Recent. Pilsbry (1896, p. 96) has noted that L. The Pecos Valley deposits are notably bulimoides is capable of considerable lacking in small species of Lymnaea: resistance to dessication and cites an ex- neither Lymnaea parva nor L. dalli seems ample in which many individuals survived to be living in Pecos Valley region today, after being packed in dry cotton for 45 although adequate studies of the living days. molluscan fauna are lacking.

LYMNAEA BULIMOIDES COCKERELLI Pilsbry

PI. IV, fig. 5 LYMNAEA PALUSTRIS (Muller) At Locality 6, south of Pyote in Ward PL IV, fig. 2 County, more than 60 specimens assignable Lymnaea palustris was found at Locali- to this subspecies were collected. The ties 3. 6. 8, 12. and 26 (fig. 41. The species temporal distribution extends only from was nowhere numerous, but the wide dis- late Pleistocene to Recent. It is a common tribution of localities of occurrence indi- living gastropod in several Great Plains cates that it was once a common inhabitant states, where it thrives in more or less of the Pecos Valley. It does not. of course, ephemeral pools. It has not been found occur in the living fauna of the region but living in the Pecos River valley. L. b. is widely distributed in North America cockerelli is obviously adapted to with- from Canada to New Mexico and in the stand long periods without open water, Northern Hemisphere generally. It is a although it occasionally occurs in peren- highly variable animal and most of the nial ponds. It seems never to occur in subspecies names applied to it seem to streams. reflect little more than local variation.

LYMNAEA CAPERATA S As a fossil, Lymnaea palustris occurs from the Kansan to Recent: Frye and PL IV, fig. 1 Leonard (1957, fig. 7) reported it from Lymnaea caperata occurs at Localities Kansan localities in Garza and Lubbock 3, 5, 9, and 26; the species was especially counties, Texas. Pleistocene Molluscan Faunas, Pecos Valley, Texas 21

Family PLANORBIDAE Genus PLANORBULA Haldeman ARMIGERA (Say) Genus HELISOMA Swainson PLANORBULA figs. HELISOMA ANTROSA (Conrad) PI. 11, 10-12 of Planorbula armi- PI. 11, fig. 3, 4 A single specimen gera was found at Locality 9, 3 miles south- Helisoma antrosa was found at six locali- west of Grandfalls, Ward County, where it ties (nos. 13,18, 22, 25, 30, 31) but was no- occurred with other aquatic pulmonate where numerous, no more than six examples gastropods as well as Calipyrgula pecosen- having been recovered at any single lo- sis and an assemblage of terrestrial snails. cality. For this reason, no attempt is made Planorbula armigera has a wide distribu- to assign these specimens to a named va- tion in North America, from northern riety, as limits of variation cannot be de- Canada to Louisiana and Georgia, but is termined. Living H. antrosa is widely dis- not known west of the Rocky Mountains. It tributed in the northern part of the United is a typical inhabitant of small pools and States and Canada and occurs with less marshes. If our analysis of the ecology of frequency in middle and southern parts of deposition of Surface IV sediments is rea- the United States. Fossils of H. antrosa oc- sonable, P. armigera should have thrived cur from early Pleistocene to Recent and in these back swamp situations, and it is are common in lower Pleistocene deposits not known why no more shells were found. on the Great Plains (Leonard, 1950; Frye and Leonard, 1957; Taylor, 1960). PLANORBULA CRASSILABRIS (Walker) H. antrosa occurs in large and small PI. 11, figs. 13-15 streams and seems to be especially adapted At Locality 8 southwest of Grandfalls, to but in northern United States creeks, it Ward County, was found a single example lakes. occurs in deep of Planorbula crassilabris; two examples were recovered from the deposits at Lo- cality 13, at Toyah, Reeves County. At each HELISOMA TRIVOLVIS (Say) of these localities P. crassilabris was asso- PL 11, figs. 1, 2 ciated with unionid mussels, numerous sphaeriids, and several species of aquatic Helisoma trivolvis the largest planor- is pulmonate gastropods. bid snail occurring in either living Texas, Within the aperture (in old individuals, or fossil. In the present study it was found sometime deeply within) of both P. armi- only at Locality 13, at Toyah in Reeves gera and P. crassilabris is to be found a County, but it has been reported the in series of excrescences or lamellar denticles. High Plains region Texas and elsewhere of The two species differ in the characters of on the High Plains (Leonard, 1950; Frye these lamellae;- crassilabris is a smaller and Taylor, 1960) as a Leonard, 1957; snail than armigera but the reproductions common Pleistocene fossil. The living on Plate 11, not being to the same scale, species is widely distributed in North give the illusion that this is not the case. America, especially in the northern half, The living species is known from Michigan, although either this or some closely related lowa, and neighboring states where it oc- species is common in the southern states. curs in ponds and small pools. Helisoma trivolvis prefers quiet, even water stagnant but is found in the quieter Genus PROMENETUS Baker of swift streams. It seems to have parts PROMENETUS UMBILICATELLUS (Cockrell) great ability to resist periods of dessication, PI. 11, figs. 19-21 and large populations may appear in road- side pools and other small and ephemeral A few shells assigned to this species were bodies of water. collected at each of the two localities (nos. 22 Report of Investigations—No. 45

6 and 24), the first in Ward County, the spicuous element of the aquatic pulmonate other in Pecos County. P. umbilieatellus is fauna of the West Indies. T. obstructus known from late Pliocene to Recent de- lives in southern Texas and Louisiana posits in the High Plains; the living form along the humid coastal areas but is not is primarily distributed in the northern now known to be living in the Pecos Valley part of central United States and in region. Canada. It lives in quiet waters, especially The three specimens collected resemble those in which there is a good growth of closely the subspecies T. o. donbilliof Tris- aquatic vegetation. It has been reported tam, but the lack of good series of shells is from the southern High Plains by Leonard basis for not assigning these individuals to (1950, p. 18) and by Taylor (1960, p. 60), a subspecies. and an unassigned species of Promenetus was reported by Frye and Leonard (1957) Family ANCYLIDAE Menke from the High Plains region of western Texas. Genus FERRISSIA Walker FERRISSIA PARALLELA (Haldeman) Genus GYRAULUS Charpentier PL 111, figs. 16, 17 GYRAULUS PARVUS (Say) were PI. 11, figs. 22-24 Shells of this ancylid gastropod col- lected at Localities 4, 6, 7, 8, 9, 26, and 28, This tiny planorbid snail occurred in the number of individuals varying from nine widely distributed localities (nos. 1, 2, one to ten at different collecting stations. 6,10,13,14, 29, 30, 31) but was not abun- The species has been reported as a fossil represented at individual out- dantly any from many localities on the Great Plains crop. Gyraulus parvus has been reported (Leonard, 1950; Taylor, 1960) from early a from late Pliocene as fossil to Recent in Pleistocene to Recent deposits. the southern Great Plains (Taylor 1960, The living form of the species is dis- p. 58). tributed from southeastern Canada to the The living species is an inhabitant of central part of the United States but is not quiet waters, such as small ponds or the known to be living in Texas. It is adapted quieter edge situations in streams. It is to living in quiet waters, where it lives on most often associated with heavy growth of the smooth stems of plants or upon old aquatic plants. It is not now living in bottles and metal cans. Texas, but occurs widely east of the Rocky Mountains from Alaska and northern Can- ada to Florida. FERRISSIA RIVTJLARIS (Say) PI. 111, figs. 20, 21 Genus TROPICORBIS Piisbry and Brown Ferrissia rivularis was found at seven TROPICORBIS OBSTRUCTUS (Morclet) localities (nos. 4, 5, 7, 9, 27, 28, and 30); PI. 11, figs. 16-18 it was most abundant at Locality 9, south- Two specimens of Tropicorbis obstruc- west of Grandfalls, Ward County where a tus were found in Surface IV sediments at large series of individuals was collected. Locality 29 on the Dunlap ranch about 12 Fossil shells have been reported from late miles south-southeast of Sheffield, Terrell Pliocene (Taylor, 1960, p. 61) to Recent County, and a single individual was recov- deposits, but shells are not common in the ered from the deposits at Locality 26, west Great Plains region in deposits younger of Fort Stockton, Pecos County. than early Pleistocene, except for those The genus Tropicorbis is widely dis- reported here. tributed from southern United States Ferrissia rivularis lives in streams on through Central America and South Ameri- stones and other smooth hard objects, such ca as far south as Argentina. It is a con- as the shells of unionid mussels. Pleistocene Molluscan Faunas, Pecos Valley, Texas 23

FERRISSIA SHIMEKI Pilsbry a stock introduced into a new environment PL 111, figs. 18,19 may change in a few generations so much to be almost unrecognizable. This tiny ancylid occurred at Localities as 7, 9, and 28 and was notably abundant at Locality 9, 3 miles southwest of Grand- Order STYLOMMATOPHORA falls, Ward County. The large series of Family POLYGYRIDAE specimens available and the unique char- Genus POLYGYRA Say acters of this minute shell leave ancylid no POLYGYRA TEXASIANA (Moricand) doubt of its identity, although it was a sur- PI. 111, figs. 7-9 prise to find it in the Pecos River valley. To the writers' knowledge F. shimeki has not Polygyra texasiana occurs at about half been previously reported as a fossil, the the localities studied but was not abundant nearest locality of living colonies being in at any of them. Our specimens are closely central Kansas. similar to the subspecies texasensis of Pils- Ferrissia shimeki is an inhabitant of bry (Pilsbry, 1940, p. 619) but lack of quiet waters, such as the edges of small long series did not permit us to assess indi- ponds where it may be found on stones, old vidual variation as we should have desired. bottles, and other smooth, hard objects, oc- The subspecies texasensis occurs in Val casionally on cattails, and often on smooth Verde County in the lower reaches of Pecos leaves, such as those of the cottonwood. River valley. According to Pilsbry (1940, Large populations may sometimesbe found p. 20), this snail lives under the fallen in lily ponds in gardens near homes or in leaves of yucca or beneath prostrate yucca public parks, although the means by which plants. Although not numerous, because of these colonies become established is not its relatively large size, it forms a conspicu- clear. It is likely that they are dispersed in ous element of the terrestrial aspect of commerce by the sale of water lilies and Pecos Valley faunas. other aquatic ornamentalplants. Polygyra texasiana was reported from late Kansas deposits in Woodward County,

Leonard 35) ; this Family PHYSIDAE Dall Oklahoma, by (1950, p. may be the same species as those described PHYSA ANATINA Lea by Taylor (1960, p. 82) from southwest- PI. 111, fig. 22 ern Kansas as Polygyra rexroadensis, but Physa anatina occurs at a majority of Taylor's specimens have not been.seen by the localities reported here (fig. 4), and at the writers. some of them in huge numbers. The species currently inhabits the southern High Family BULIMULIDAE

Plains and is common in and around stock Genus BULIMULUS Leach tanks in the Pecos region. It also Valley BULIMULUS DEALBATUS DEALBATUS (Say) occurs in the Pecos River, especially in PI. IV, fig. 6 quiet water near the border of the stream or where there are mats of vegetation. It is Bulimulus dealbatus dealbatus is es- also fairly common in small tributary sentially a southern snail, but it ranges as creeks, including those that are usually dry far north as north-central Kansas. To our for several months each year. knowledge, it is not known living in the As a fossil, Physa anatina occurs in the Pecos Valley region but is common in southern High Plains from late Pliocene to central Texas. In our collections it occurs, Recent deposits. Identification of fossil never in large numbers, at Localities 8, shells is often hazardous, inasmuch as it is 13, 28, 31, and 32, which would indicate known that the living forms respond readi- from its distribution that it was once com- ly to local environmental conditions, and mon in Pecos Valley. 24 Report of Investigations—No. 45

Bulimulus d. dealbatus is well adapted in deposits in the Great Plains region, to arid climates, although it is not confined ranging from early to late Pleistocene in to them. In times of drouth it seals the age, but since these internal shells bear aperture of the shell with mucous to some few characters, it is not possible to say object, such as the stem of a plant, and whether all are actually those of the same will survive many months in this position. species, as no living slug is known to which the shells may be referred. BULIMULUS ALTERNATUS (Albers) MARIAE Shells assigned to Deroceras PI. IV, fig. 7 aenigma occur at Localities 7,10, and 30. Although The figured specimen is from Kinney nothing is known of the animal, it may be County; no fossils of Bulimulus alternatus presumed that it lived in moist situations mariae were found in our collections in the of the kind to which the naked and un- Pecos River valley region, but inasmuch protected slugs are adapted. as living colonies occur in central Val Verde County, it is likely that the species Family ENDODONTIDAE will be found as a late Pleistocene fossil in Genus HELICODISCUS Morse the region. It is a conspicuous snail, often HELICODISCUS PARALLELUS (Say) more than inch in length. The living an PI. 111, figs. 1-3 individuals observed in Val Verde County were an almost immaculate white. Helicodiscus parallelus is another of the minute terrestrial gastropods widely dis- Family ZONITIDAE tributed over North America and capable Genus HAWAII A Gude of situations too HAWAHA MINUSCULA (Binney) surviving ecological rigor- PI. 111, figs. 10-12 ous for most snails. It is one of the few snails that can live in the high, dry grass- Hawaiia minuscula is one of the most lands in the more arid parts of the Great übiquitous gastropods of fossil and living Plains, but it occurs more abundantly molluscan faunas on the Great Plains, and beneath decaying logs and in leaf mold the same is true in the Pecos Valley. The beneath patches of trees or shrubs. species occurs at more than half of the Although Helicodiscus parallelus is localities studied, at some of them in large usually a common fossil in deposits rang- numbers. H. minuscula is distributed over ing in age from middle to late Pleistocene, much of North America and is found in it was found only at Localities 8 and 31, the West Indies. It is adaptable to a wide at each of which it was represented by a range of habitat situations and is one of few individuals. the rather limited number of terrestrial The shell is easily recognized in spite gastropods that survived the period of ex- of its small size by its coin shape and termination of many gastropods on the conspicuous parallel raised striations. Great Plains toward the end of the Pleisto- cene. Living specimens were not found in HELICODISCUS EIGENMANNI Pilsbry Pecos River valley, fresh shells but were PI. 111, figs. 4—6 seen in drift in Pecos River. Hawaiia minuscula is known in the Helicodiscus eigenmanni is represented our Great Plains region as a fossil in early in collections only by two shells col- lected at Pliocene to late Pleistocene deposits. Locality 30, where it occurs in Surface IV sediments in the channel banks Family LIMACIDAE of Pecos River on the Chandler ranch in Genus DEROCERAS Terrell It is a common of DEROCERAS County. element AENIGMA Leonard the fauna of PI. 11, fig. 27 living gastropod Texas and Mexico, although it ranges as far north The object figured is the internal shell as Colorado. The shells of H. eigenmanni of a slug. Similar shells have been found resemble those of H. parallelus but are Pleistocene Molluscan Faunas, Pecos Valley, Texas 25 usually more robust, and the internal of the shells reported to be grosvenori denticles, which are usually too deeply actually belong to that species. placed to be observed in the shells of Members of the genus Succinea are often parallelus, are situated (when present) found in moist situations, but they are by near the aperture. no means restricted to such habitats. We Living colonies of H. eigenmanni are have found them in Texas, aestivating on known from the vicinity of Devils River rocky cliffs, far from water and in open in Val Verde County, but no living indi- sunlight where the local temperature was viduals were observed in the middle extremely high. Such aestivating indi- reaches of Pecos River valley. viduals can be activated months later by placing them in a moist chamber. Family SUCCINEIDAE SUCCINEA LUTEOLA Gould Genus OXYLOMA PL IV, fig. 9 OXYLOMA sp. At four localities (nos. 11, 12, 22, 32) PL IV, fig. 11 shells closely resembling those of Succinea A single, badly damaged shell that is lutelola occurred in our collections. As judged to belong to some species of Oxy- noted above, there is little assurance that loma was found at Locality 6, in western these shells belong to the species of which Ward County near the Pecos River. Even the name is here used. S. luteola is known, the generic assignment is purely tentative. however, in colonies on the high land west of Devils River, Val Verde County, and Genus SUCCINEA Draparnaud in arid places in Arizona and New Mexico The species in the genus Succinea and (Pilsbry, 1940, p. 828), and it may be related genera in the family Succineidae that the shells reported here are actually can be diagnosed only with accuracy by S. luteola. means of characters associated with the SUCCINEA AVARA Say It is well known that the soft anatomy. PL IV, fig. 10 shells are notoriously unreliable as clues to species. Therefore, in the following three At eleven localities (fig. 4) shells of accounts, the use of specific names is de- small Succinea resembling those of Suc- were almost signed only to give some concept of the cinea avara collected; it is a size and form of the shell; it is entirely certainty that these shells constitute a com- probable that none of these actually belong plex of species, because it is well known to the species whose names are used with that several of the smaller kinds that re- them. semble avara as far as the shell is con- cerned, are anatomically different enough SUCCINEA GROSVENORI Lea to constitute not only different species but PI. IV, fig. 8 different genera of Succineidae (Miles, 1958). However, small shells of this sort Shells resembling in size and general are numerous in many early to late Pleisto- form those of recognized Succinea gros- cene deposits in the Great Plains region of venori were found at eleven localities (fig. western Texas and elsewhere on the Great 4). shells occur on the GreatPlains Similar Plains farther north. as fossils in deposits ranging in age from Nebraskan to late Pleistocene and geo- Family PUPILLIDAE graphically from western lowa to Texas Genus GASTROCOPTA Wellaston to (Leonard, 1950, p. 24). According GASTROCOPTA PENTODON (Say) Pilsbry (1940, 819), the species ranges p. PL 111, fig. 29 widely over North America, but since few anatomical studies have been made, there A single specimen of Gastrocopta pento- is room for considerable doubt that many don was found at Locality 9, in sediments 26 Report of Investigations—No. 45 beneath Surface IV, 3 miles southwest of Grandfalls, Ward County. Although G. CASTROCOPTA CRISTATA (Pilsbry and Vanatta) pentodon ranges from eastern United States and Canada to Mexico through a PI. 111, fig. 26 wide variety of habitat situations, the nar- Gastrocopta cristata occurred at nearly row band of humid ecological conditions half of the localities studied (fig. 4) and near the Pecos River in post-Twocreekan at many of them was found in abundance. time seems to have been unsuitable for it. G. cristata is a common Pleistocene fossil G. pentodon generally inhabits uplands, in the Great Plains region. It occurs in late and most often timbered areas. Kansan deposits distributed from western It is not well known as a fossil, but Frye lowa to southwestern Kansas (Leonard, and Leonard (1957, fig. 9) reported it 1950, p. 32) and was reported from many from several High Plains localities of early localities along the escarpment of the High Wisconsinan deposits in western Texas. Plains region of Texas by Frye arid Leon-

ard (1957) ; stratigraphically, it ranges there from Kansan to late Pleistocene. G. GASTROCOPTA PROCERA PROCERA (Gould) cristata is a part of the gastropod fauna of PI. 111, fig. 27 southwestern United States; it is highly adapted to arid conditions. A few specimens of Gastrocopta procera having the characters of the typical species Genus PUPOIDES Pfelffer were collected at Localities 17 and 30; it PUPOIDES ALBILABRIS (Adams) occurred at both with Gastrocopta stations PI. 111, fig. 25 cristata, which was much more abundant. G. procera is not a common Pleistocene Pupoides albilabris, one of the largest fossil. Frye and Leonard (1957) did not pupillid snails of the region, was found at report it from their studies along the High 12 localities among the 33 studied (fig. Plains escarpment; however, Leonard 4). The species is widely distributed east (1950, p. 32) reported the species from of the Rocky Mountains and in the West late Kansan localities in central Kansas Indies. Although elsewhere it occupies and in northwestern Oklahoma, and Taylor various habitat situations, including some (1960, p. 67) listed G. procera from early very humid ones, P. albilabris is one of the Pleistocene localities in southwestern few common snails in the short-grass en- Kansas. vironment on the Great Plains, where it The living form of the species is dis- endures long periods of severe dessication tributed widely' in the southern part of the and extremely high temperatures. United States, a little farther north than P. albilabris is a common fossil in the the general latitude of the Ohio River. Its Great Plains region, where it occurs in habitat is usually situated in woodlands, deposits ranging in age from lower Plio- but a small clump of trees is sufficient to cene to late Pleistocene (Leonard, 1950, harbor a colony. p. 29; Taylor, 1960, p. 74). The shell of this species, especially in western popula- tions, is heavy and ruggedly constructed, GASTROCOPTA PROCERA MCCLUNGI(?) which probably improves its chances of (Hanna and Johnston) being preserved as a fossil. PI. 111, fig. 28 Genus PUPILLA Leach Only two individuals assignable to this PUPILLA BLANDI Morse subspecies were found, one each at Locali- ties 13 and 26. Inasmuch as the shells were PI. 111, fig. 23 not well preserved, the identity of these Pupilla blandi occurs in our collections shells is doubtful. at Locality 6 and Locality 13; it was es- Pleistocene Molluscan Faunas, Pecos Valley, Texas 27

pecially numerous at Locality 6 in early the occurrence of V. gracilicosta in Kansan Wisconsinan sediments near the Pecos deposits, and Frye and Leonard (1957) River in western Ward County. Along with reported its occurrence in numerous Pupilla muscorum, it is a common loess Pleistocene localities along the High Plains fossil in the more northern Great Plains escarpment, where the deposits ranged and in the Mississippi Valley. Also, like from Kansan to early Wisconsinan. V. Pupilla muscorum, P. blandi was elimi- gracilicosta vanished from most of the nated from Great Plains molluscan faunas GreatPlains region after Twocreekan time, justbefore or during the Bradyan interval. although it occurs in northern Nebraska It ranges from Kansan deposits to early (Taylor, 1960, p. 77) and elsewhere to Wisconsinan deposits generally over the the northwest. In the montane areas it Great Plains region, from western lowa to ranges down to New Mexico, and to the Texas. northeast into Minnesota (Pilsbry, 1948, p. 1028). PUPILLA MUSCORUM (Linne) PI. 111, fig. 24 AGE AND CORRELATION OF FOSSIL shells of A moderately good series of MOLLUSCAN FAUNAS Pupilla muscorum was recovered from deposits in northern Crane County (Lo- No previous attempts have been made cality 1). P. muscorum, an old-world to study in an organized and systematic species that first appears in the Great manner the stratigraphic paleontology of Plains region in Kansan deposits, is widely the late Cenozoic in the Pecos River valley distributed as a fossil from western lowa region of Texas. This fact, the lack of de- to southwestern Texas. As noted above, it tailed topographic maps for theregion, and does not occur in late Pleistocene deposits the relatively short time that was available in this geographical region. The strati- for field study, make certain aspects of graphic and geographic distribution of P. this report little more than a reconnais- muscorum has been discussed by Leonard sance. Nevertheless, we judge that several

(1950, p. 28) ; Frye and Leonard (1957) conclusions are justified by the facts avail- reported this fossil from numerous locali- able. ties in their studies of Pleistocene fossil- Although no fossil mollusks were col- iferous deposits of the High Plains region lected from beneath Surface I, we are con- of western Texas. vinced that it is late Tertiary in origin because (1) it is graded to the drainage- Family VALLONIDAE way that deposited the graveliferous sedi- Genus VALLONIA Risso ments of the Ogallala formation on the of the VALLONIA GRACILICOSTA Reinhardt south flank High Plains, capping in many places the east bluff of Pecos River PI. 11, figs. 25, 26 valley (PI. I, A, D, F) -, (2) it forms the Vallonia gracilicosta was found at three highest erosional surface below the Cre-

localities in the Pecos Valley region (nos. taceous uplands (PI. I, E) ; and (3) the 1, 2, and 6). This species in the mid- collection of fossil mollusks at Locality 33 continent region is one of the most numer- shows definitely that the next lower surface ous and widespread of molluscan fossils, (Surface II) is Pleistocene. Surface I de- from Nebraskan to early Wisconsinan, but veloped as an extensive pediment during does not occur in post-Twocreekan sedi- latest Tertiary, and although locally dis- ments in the region. Its occurrence in the sected and somewhat lowered during early Pecos Valley region is, however, much less Pleistocene time, it is now well graded and widespread and populations much less veneered with colluvium except for widely abundant than is true farther north on the spaced intermittent drainageways. We High Plains. Leonard (1950) discussed judge Surface I, although somewhat low- 28 Report of Investigations—No. 45 ered, to be a relatively intact late Tertiary explain the presence of sluggish streams surface. and/or ponds in this situation persisting Inasmuch as the shells at Locality 33 from the time that local drainage was inte- come from alluvial sediments underlying grated with the Kansan Pecos until the Surface 11, it is demonstrated that it is time that the Pecos had reached its early Pleistocene in origin, and since the fossil Wisconsinan level. molluscan faunas from the alluvial sedi- Correlation of fossil molluscan faunas ments underlying Surface 111 are charac- in alluvial sediments beneath Surface 111 teristic early Wisconsinan assemblages, it on the southwestern flank of the Pecos follows that the cutting of Surface II repre- River valley with fossil molluscan as- sents a complex history that probably semblages recovered from terrace rem- extended through all Pleistocene time to nants on the northeastern side of the valley the end of the Kansan stage. Climatic de- are much more certain, and there is, more- terioration in the region in Illinoian time over, little doubt that these faunas are early is judged to have eliminated an integrated Wisconsinan. As examples of molluscan drainage pattern from this surface and faunal assemblages that bear out this developed the extensive coalescent pedi- judgment we cite those from Locality 13 ments and their surficial veneers to es- near Toyah, Reeves County, and the as- sentially their present form. Surface 11, semblage from Locality 6 in western Ward although perhaps slightly lowered, has County. The assemblages are typologically been essentially stabilized since its de- similar, both reflect essentially the same velopment during the Illinoian. ecological conditions, and both contain The sediments at Localities 1 and 2 in representatives of the genus Pupilla, char- northern Crane County are here tentatively acteristic of early Wisconsinan faunas and correlated Surface with those underlying absent from post-Twocreekan sediments. 11. The molluscan faunas do not offer con- Furthermore, the deposits at Locality 6, clusive evidence on this point. However, except for lack of colluvial a synthesis of the available data places the veneer, corre- late the weight of the evidence in favor of these topographically with general level sediments being Kansan in age. The ele- of Surface 111 and exist in the form of a ments of the argument are as follows: (1) terrace, distinctly distinguishable physio- although the fossil molluscan fauna is not graphically from the terrace here desig- conclusively characteristic of Kansan as- nated Surface IV. semblages, there are no faunal elements Fossil molluscan faunas contained in the that exclude it from such an assemblage; sediments below Surface IV, except for the (2) the sediments from which the fossils Recent mollusks in the overwash sediments were collected occur in a topographic above the weakly developed soil, are cer- situation consistent with an alluvial valley tainly latest Wisconsinan and judged to fill from the north, graded to the same local be post-Twocreekan in age. This age as- base level that controlled and Surface II; signment is based on topographic position, (3) the deposits in which the fossils are the terrace being the lowest and obviously incorporated are sandy marls, indicating a the the Pecos River well-watered local contemporary terrain. youngest present in valley; the unweathered state of The deposits at Localities 1 and 2 do not relatively fit the conditions which almost certainly the deposits; and the rich molluscan fauna prevailed during the extended period of contained in it, that lacks representatives arid climate known to have characterized of Pupilla and other kinds of mollusks the Illinoian stage in the region. Finally, known to have disappeared from the south- local physiographic relations seem to ex- ern Great Plains and adjoining regions clude these deposits from being early Wis- before the deposition of post-Twocreekan consinan, as it is extremely difficult to terraces. Pleistocene Molluscan Faunas, Pecos Valley, Texas 29

REFERENCES

BAKER, F. C. (1911) The Lymnaeidae of North MILES, C. D. (1958) The family Succineidae and Middle America: Chicago Acad. Sci. Spe- (Gastropoda: Pulmonata) in Kansas: Univ. cial Pub. No. 3, 539 pp., 58 pis., 51 figs. Kansas Sci. Bull., vol. 38, pp. 226-240, 2 pis., 2 FENNEMAN, N. M. (1931) Physiography of West- figs. ern United States: McGraw-Hill Book Co., New PILSBRY, H. A. (1896) Limnaea bulimoides Lea York, 534 pp. resisting drouth: Nautilus, vol. 10, p. 96. and (1957) Studies of FRYE, J. C., LEONARD, A. B. (1940) Land Mollusca of North Amer- Cenozoic geology along eastern margin of Texas ica: Acad. Nat. Sci. Philadelphia, Monograph High Plains, Armstrong Howard counties: to No. 3, vol. 1, pt. 2, pp. 575-994, figs. 378-580. Univ. Texas, Bur. Econ. Geol. Rept. Inv. No. 32, 62 pp. (1948) Land Mollusca of North Amer- (1957 a) Ecological ica: Acad. Nat. Sci. Philadelphia, Monograph ( interpre- tations of Pliocene and Pleistocene stratigraphy No. 3, vol. 2, pt. 1, pp. 521-1113, figs. 282-581. in the Great Plains region: Amer. Jour. Sci., SELLARDS, E. H., ADKINS, W. S., and PLUMMER, vol. 255, pp. 1-11. Reprinted as Univ. Texas, F. B. (1933) The geology of Texas, Vol. I, Bur. Econ. Geol. Rept. Inv. No. 29. Stratigraphy: Univ. Texas Bull. 3232, Aug. 22, (1959) Correlation of the 1932, 1007 pp. , (Neogene) Texas Ogallala formation in western (1960) Late Cenozoic molluscan Nebraska: Texas, TAYLOR, D. W. with type localities in Univ. faunas from the High Plains: U. S. Geol. Survey Rept. Bur. Econ. Geol. Inv. No. 39, 46 pp. Prof. Paper 337, 94 pp., 4 pis., 2 figs., 19 tables. LEONARD, A. B. (1950) A Yarmouthian molluscan fauna in the mid-continent region of the United States: Univ. Kansas Paleo. Contrib., MOL- LUSCA, art. 3, 48 pp., 6 pis., 4 figs.

Plates I-IV 32 Report of Investigations—No. 45

PLATE I Some geologic features of the Pecos River valley A. View northeast across Pecos River, 3.5 miles northeast of Imperial, in Crane County, Texas; fossil Locality 4 (1958). Fauna collected from deposits under Surface IV in middle distance; Triassic rocks exposed in channel of Pecos River below fossiliferous sediments. Northeast bluff of Pecos Valley, in distance, capped by Ogallala formation, and Triassic rocks are exposed in lower part of bluff. B. Cemented colluvium of early Wisconsinan pediment veneer, 2 miles below Pandale in Pecos River valley, Val Verde County, Texas (1958). C. Cut bank along Salt Draw, 4]/2 miles northeast of Toyah, Reeves County, Texas (1958). Early Wisconsinan snail fauna occurs in the deposits below Surface III; fauna is similar to that col- lected farther south at Locality 13. The deposit consists of gray clayey silts and sands, with a few thin lenses of gravel. Surface 111 here is a narrow terrace below the truncated edge of Surface II that yielded snails farther south. Locally, a very narrow terrace of Surface IV occurs (not shown in view) that yielded sparse Succinea. D. North across pediment Surface 111 toward Pecos River, 14 miles west of Grandfalls, in Pecos County, Texas (1958). This smooth, ungullied, colluvial veneered surface extends several miles north to within a few hundred yards of Pecos River channel where a narrow Surface IV terrace occurs at a slightly lower level. Fauna Locality 6 occurs in the truncated edge of the early Wis- consinan sediments below this surface near the river. Southward, the unbroken Surface 111 truncates Surface II with a distinct but low-angle change of slope, as shown by sections B and C on figure 2. Under the sparse cover of vegetation, unconcentrated sheet wash, although incapable of producing gullies, maintains active transit of the surficial materials. E. Pediment pass on Surface I, looking south-southeast, 2.3 miles south-southeast of Girvin, Pecos County, Texas (1958). The upland mesas are developed on Cretaeous rocks, and the high elements of pediment Surface I, as shown here, are cut surfaces on Cretaceous bedrock; this cut pediment surface grades imperceptibly downward to a veneered surface, and in northern Pecos County thin remnants of Ogallala formation occur on the lower elements of this surface. Surface I is identified throughout a vertical distance of approximately 1,000 feet (fig. 2) without physiographic discontinuity. F. Ogallala formation on Triassic rocks exposed in road cut through north valley wall of Pecos Valley, along Farm Road 1053, iy 2 miles northeast of Imperial, in Crane County, Texas (1958). Pleistocene Molluscan Faunas Plate I 34 Report of Investigations—No. 45

PLATE II Fossil mollusks from the Pecos River valley region of Texas (Magnifications are approximate)

FIGURES— PAGE 1,2. Helisoma trivolvis (Say), x3. East end of bridge, east side of Toyah, Reeves County,

Texas. Cat. no. 11214 . 21 3,4. Helisoma antrosa (Conrad), x6. Right bank of Pecos River, one-fourth mile above mouth of Independence Creek, Chandler ranch, Terrell County, Texas. Cat. no. 11314 21 5. Calipyrgula circumstriata Leonard and Ho, xB. Right bank of Pecos River, one-fourth mile above mouth of Independence Creek, Chandler ranch, Terrell County, Texas. Cat. no. 11301 - 19 6. Calipyrgula pecosensis Leonard and Ho, xB. Three and one-half miles northeast of Im-

perial, in Crane County, Texas, left bank of Pecos River. Cat. no. 11266 .. 18 7-9. Cochliopa riograndensis Pilsbry and Ferriss, xB. Right bank of Pecos River, one-fourth mile above mouth of Independence Creek, Chandler ranch, Terrell County, Texas. Cat. no. 11343 19 10-12. Planorbula armigera (Say), x6. Left bank of Pecos River, 3 miles southwest of Grand- falls, Ward County, Texas. Cat. no. 11319 21 13-15. Planorbula crassilabris (Walker), xB. East end of bridge, east side of Toyah, Reeves County, Texas. Cat. no. 11217 21 16-18. Tropicorbis obstructus (Morelet), x6. W. C. Dunlap, ranch, 12 miles south-southeast of Sheffield, Terrell County, Texas. Cat. no. 11303 22 19-21. Promenetus umbilicatellus (Cockerell), x6. Fourteen miles south-southwest of Rankin, Pecos County, Texas, near bridge of State Highway 349. Cat. no. 11144 21 22-24. Gyraulus parvus (Say), xB. Cut bank east side of U.S. Highway 80, IV2 miles south of Toyah, Reeves County, Texas. Cat. no. 11200 22 25,26. Vallonia gracilicosta Reinhardt, xB. Seven-tenths mile north of junction of Farm Roads 1601 and 1233, on Farm Road 1601, Crane County, Texas. Cat. no. 11159 27 27. Deroceras aenigma Leonard, x6. Left bank of Pecos River, 1 mile east of Pecos, in Ward County, Texas. Cat. no. 11304 24 Pleistocene Molluscan Faunas Plate II 36 Report of Investigations—No. 45

PLATE III Fossil mollusks from the Pecos River valley region of Texas (Magnifications are approximate) FIGURES— PAGE 1-3. Helicodiscus parallelus (Say), xB. Twenty miles south of Sheffield, in Terrell County, Independence Texas, at State Highway 349 crossing of Creek. Cat. no. 11132 .. 24 4—6. Helicodiscus eigenmanni Pilsbry, xB. Right bank of Pecos River, one-fourth mile above mouth of Independence Creek, Chandler ranch, Terrell County, Texas. Cat. no. 11313 24 7-9. Polygyra texasiana (Moricand), x3. Right bank of Pecos River, one-fourth mile above mouth of Independence Creek, Chandlei ranch, Terrell County, Texas. Cat. no. 11324 23 10-12. Hawaiia minuscula (Binney), xB. Right bank of Pecos River, one-fourth mile above mouth of Independence Creek, Chandler ranch, Terrell County, Texas. Cat. no. 11312 24 13-15. Helicina orbiculata tropica Pfeiffer, x3. Seven miles west of Brackettville, Kinney County, Texas. Cat. no. 11112 19

16,17. Ferrissia parallela (Haldeman), x6. Left bank of Pecos River, 1 mile east of Pecos, in

Ward County, Texas. Cat. no. 11305 —. 22 18,19. Ferrissia shimeki Pilsbry, xB. Three miles southwest of Grandfalls, Ward County, Texas. Cat. no. 11307 23 20,21. Ferrissia rivularis (Say), x6. Left bank of Pecos River, 1 mile east of Pecos, in Ward County, Texas. Cat. no. 11306 22 22. Physa anatina Lea, x2. Left bank of Pecos River, 100 yards below bridge on Farm Road 305, 12 miles south-southwest of McCamey, Crockett County, Texas. Cat. no. 11317 23 23. Pupilla blandi Morse, xB. One and one-half miles south of Toyah, cut bank along east side of U.S. Highway 80, in Reeves County, Texas. Cat. no. 11198 26 24. Pupilla muscorum (Linne), xB. Caliche pit, 7.8 miles north of junction of Farm Roads 1601 and 1233 in Crane County, Texas. Cat. no. 11231 27 25. Pupoides albilabris (Adams), xB. Right bank of Pecos River, one-fourth mile above mouth of Independence Creek, Chandler ranch, Terrell County, Texas. Cat. no. 11321 26 26. Gastrocoptacristata (Pilsbry and Vanatta), xB. Pecos River at U.S. Highway 290 bridge, 3 miles southeast of Sheffield, Pecos County, Texas. Cat. no. 11308 26 27. Gastrocopta procera procera (Gould), xB. Eleven miles east-southeast of junction of U.S. Highway 290 and State Highway 17, on Barilla Creek, Reeves County, Texas. Cat. no. 11102 26

28. Gastrocopta procera mcclungi (?) (Hanna and Johnston), xB. East end of bridge, east side of Toyah, Reeves County, Texas. Cat. no. 11215 26 29. Gastrocopta pentodon (Say), xB. Three miles southwest of Grandfalls, Ward County, Texas. Cat. no. 11309 25 Pleistocene Molluscan Faunas Plate III 38 Report of Investigations—No. 45

PLATE IV Fossil mollusks from the Pecos River valley region of Texas (Magnifications are approximate)

FIGURES— PAGE

1. Lymnaea caperata Say, x6. Six and one-half miles west of junction of U.S. Highway 290-67 with U.S. Highway 285, Fort Stockton, Pecos County, Texas. Cat. no. 11187 20 2. Lymnaea palustris (Miiller), x3. Two miles northwest of U.S. Highway 285 railroad

crossing, Pecos, Reeves County, Texas. Cat. no. 11208 - 20 3. Lymnaea parva Lea, x6. Six and one-half miles west of junction of U.S. Highway 290-67 with U.S. Highway 285, Fort Stockton, Pecos County, Texas. Cat. no. 11185 20 4. Lymnaea bulimoides bulimoides Lea, xB. Three miles southwest of Grandfalls, Ward County, Texas. Cat. no. 11316 20 5. Lymnaea bulimoides cockerelli (Pilsbry and Ferriss), x 3. Seven-tenths mile north of Pecos River channel, extension of Farm Road 1927 in Ward County, Texas. Cat. no. 11169 20 6. Bulimulus dealbatus dealbatus (Say), x2. Two miles north of road junction in Pandale, on Ozona road, Val Verde County, Texas. Cat. no. 11155 23 7. Bulimulus alternatus mariae (Albers), x2. Seven miles west of Brackettville, Kinney County, Texas. Cat. no. 11108 24 8. Succinea grosvenori Lea, x 3. East end of bridge, east side of Toyah, Reeves County, Texas. Cat. no. 110% 25 9. Succinea luteola Gould, x3. Two miles north of road junction in Pandale, on Ozona road, Val Verde County, Texas. Cat. no. 11154 25 10. Succinea avara Say, x3. Six and one-half miles west of junction of U.S. Highway 290-67 with U.S. Highway 285, Fort Stockton, Pecos County, Texas. Cat. no. 11191 25 11. Oxyloma sp., x 3. Seven-tenths mile north of Pecos River, extension of Farm Road 1927, Ward County, Texas. Cat. no. 11165 25 12. Ligumia sp., x6. Seven-tenths mile north of Pecos River, extension of Farm Road 1927, Ward County, Texas. Cat. no. 11263 18 13. Unionid, indeterminate, x6. Two and one-half miles southwest of Grandfalls, Ward County, Texas. Cat. no. 11265 18 14. Actiononais sp., x6. East end of bridge, east side of Toyah, Reeves County, Texas. Cat. no. 11264 17 15,16. Sphaerium striatinum (Lamarck), x4. Screw Bean Arroyo, one-half mile south of Red Bluff, Reeves County, Texas. Cat. no. 11174 18 17,18. Pisidium compressum Prime, xB. Five and six-tenths mile northwest of Fort Stockton, U.S. Highway 285, in Pecos County, Texas. Cat. no. 11120 18 19,20. Pisidium casertanum (Poli), xB. Three miles southwest of Grandfalls, Ward County, Texas. Cat. no. 11407 18 Pleistocene Molluscan Faunas Plate IV

Index

\ris: 17, 38 Great Plains: 21, 25 [Deroceras: 24, 34 grosvenori, Succinea: 25, 38 ulluscan faunas: 27-28 Gyraulus parvus: 22, 34 is,Pupoides: 26, 36 is, Bulimulus: 24, 38 Hawaiia minuscula: 24, 36 Physa: 23, 36 Helicina orbiculata: 19, 36 Helisoma: 21, 34 Helicinidae: 19 gastropoda: 18 Helicodiscus eigenmanni: 24, 36 t, Planorbula: 21, 34 parallelus: 24, 36 uccinea: 25, 38 Helisoma antrosa: 21, 34 trivolvis: 21, 34 Creek: 36 High Plains: 7, 10,11, 15,16,21, 22, 23 utophora: 19 Howard County: 10 r 11 Hydrobiidae: 18 Papilla: 26, 36 rr County: 9 Illinoian: 15, 28 ides, Lymnaea: 20, 38 Independence Creek: 16, 19, 34, 36 ilidae: 23 ilus alternatus: 24, 38 Kansan stage: 17, 26 lotus: 23, 38 King Mountain: 10 soils: 6 Kinney County: 36, 38 rgula circumstriata: 19, 34 Lamellibranchiata: 17 sensis: 18, 21, 34 Lampsilinae: 17 to, Lymnaea: 20, 38 Ligumia: 18, 38 mum, Pisidium: 18, 38 recta: 18 ■ler ranch: 16,19, 24, 34, 36 Limacidae: 24 striata, Calipyrgula: 19, 34 Loving County: 7, 12 opa riograndensis: 19, 34 Lubbock County: 20 tssum, Pisidium: 17, 18, 38 luteola, Succinea: 25, 38 and: 15 Lymnaea bulimoides: 20, 38 County: 7,10,12, 15, 27, 28, 32, 34, 36 caperata: 20, 38 ibris, Planorbula: 21, 34 palustris : 20, 38 [>ck:sous—•10,17,32 parva: 20, 38 ck: 10, 17, 32 Lymnaeidae: 19 e: 11 tones: 9, 10 marshes: 16 i: 7 Midland County: 10

, Gastrjcopta: 26, 36 minuscula, Hawaiia: 24, 36 tt County: 12, 36 molluscan faunas: 13-28 ofiles: 8 muscorum, Papilla: 27, 36

lountains: 7, 9, 10,14 Nebraskan stage: 17, 27 us, Bulimulus: 23, 38 as aenigma: 24, 34 Ogallala formation: 7,10, 14, 32 liver: 7, 25 drainageways: 5, 14 ranch: 34 orbiculata, Helicina: 19, 36 Oxyloma: 25, 38 ounty: 10 s Plateau: 7 palustris, Lymnaea: 20, 38 'Jini, Helicodiscus: 24, 36 parallela, Ferrissia: 22, 36 ntidae: 24 parallelus, Helicodiscus: 24, 36 parva, Lymnaea: 20, 38 i parallela: 22, 36 parvus, Gyraulus: 22, 34 ris: 22, 36 Pecos Couuty: 8, 11, 20, 22, 32, 34, 36, 38 ki: 23, 36 Pecos River and/or Valley: 5, 7,10,11, 12,13 14, 15, 16, 17, 20,23,25,27 Calipyrgula: 34 20 pecosensis, 18, 21, ounty: pedimentation: 12, 14, 15, 16 ypta cristata: 26, 36 25, pediments: 8 ion: 36 slope veneers: 13, 14 as 26, 36 colluvium, surfaces: 5, 9, 11, 32 ( see also Surfaces I-lV) >da: 16, 18 16 k County: pelecypods, sphaeriid: 10 Gastrocopta: 25, 36

Permian rocks: 7 Sphaerium striatinum: 17, 18, 38 Physa anatina: 23,36 striatinum, Sphaerium: 17,18, 38 physiography: 7-12 Stylommatbphora: 23 Pisidium casertanum: 18, 38 Succinea: 11, 14, 25, 32 compressum: 17,18, 38 avara: '25, 38 Planorbidae: 21 grosvenori: 25, 38 Planorbula armigera: 21, 34 luteola: 25, 38 crassilabris: 21, 34 Succineidae: 25 Pleistocene: 6, 20,21 Surface I, 11, 111, IV: 5, 11, 12, 14, 15, 16, 17, 18. Pliocene age: 18 19, 21, 24, 27, 28 Polygyra rexroadensis: 23 texasiana: 23, 36 Teleodesmacea: 18 Polygyridae: 23 Terrell County: 16,19, 24, 34, 36 post-Twocreekan sediments, terraces, time: 13, Tertiary Pecos River: 11 16, 28 texasina, Polygyra: 23, 36 Prionodesm'acea: 17 Trans-Pecos region: 15 procera, Gastrocopta: 26, 36 Triassic rocks: 7, 8 Promenetus umbilicatellus: 21, 34 trivolvis, Helisoma: 21, 34 Prosobranchia: 18 Tropicorbis obstructus: 22, 34 Pulmonata: 19 Twocreekan time: 16 (see also post-Twocreekan) Pupilla: 23 blandis '26, 36 umbilicatellus, Promenetus: 21, 34 muscorum: 27, 36 unionid indet.: 38 Pupillidae: 25 Unionidae: 17 Pupoides albilabris: 26, 36 Upton County: 7,10 Recent age: 12,18, 20 valley incision: 12 recta, Ligiimia: 18 Vallonia gracilicosta: 27, 34 Reeves County: 8, 11, 13, 15, 16, 17, 19, 32, 34, Vallonidae: 27 36, 38 Val Verde County: 11, 12, 13,16, 17, 25, 32, 38 rexroadensis, Polygyra: 23 volcanic ash beds: 6 riograndensis, Cochliopa: 19, 34 rivularis, Ferrissia: 22, 36 Ward County: 7, 10, 12, 16, 18, 20, 21, 22, 23, 26, 27, 34, 36, 38 Salt Draw: 32 Winkler County: 10 Screw Bean Arroyo: 38 Wisconsinan: 12, 16, 18, 27 Sheffield, Terrell County: 19 assemblages: 28 shimeki, Ferrissia: 23, 36 deposits: 18 soils, buried: 6 pediment veneer: 32 Sphaeridae: 18 sphaeriid pelecypods: 16 Zonitidae: 24