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RI0045. Pleistocene Molluscan Faunas and Physiographic History of Pecos Valley in Texas, by A. B. Leonard and J. C. Frye. 42 p., 4 figs., 4 plates, 1962. ISSN: 0082335X: Print.
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Studies of physiographic relations and of fossil molluscan faunas made in the Pecos River valley region (Val Verde County to the Texas–New Mexico border) reveal extensive, well-pedimented surfaces southwest of the river extending from the Davis Mountains to near the present channel. These surfaces are described as Surface I, graded to the late Tertiary Ogallala drainageways; Surface II, graded to a lower level early Pleistocene channel of Pecos River and extensively pedimented during Illinoian time; Surface III, graded during early Wisconsinan to a level only slightly above the present channel of Pecos River; and Surface IV, developed during latest Wisconsinan and Recent as a terrace of highly fossiliferous deposits in which the Pecos River is presently incised. No molluscan fossils were observed in the sediments of Surface I; meager collections in alluvial sediments underlying Surface II confirm its Pleistocene (?) age; adequate collections of fossil mollusks from alluvial sediments beneath Surface III are characteristic of early Wisconsinan, and the uniquely rich faunas in deposits below Surface IV are characteristic of post- Twocreekan (post-Bradyan) assemblages.
Physiographic features of Pecos River valley are illustrated by generalized profiles and photographs; the 33 localities from which fossil mollusks were collected are indicated on a location map, the species included in each local molluscan faunal assemblage are shown on a chart, and each species is illustrated photographically.
Keywords: Pecos River valley, Val Verde County, physiography, fossils, mollusks, Wisconsinan, Texas
Leonard, A. B., and Frye, J. C., 1962, Pleistocene Molluscan Faunas and Physiographic History of Pecos Valley in Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 45, 42 p.