Browsing by Subject "Mollusks, Fossil -- Pecos River Valley (New Mexico and Texas)"

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Pleistocene molluscan faunas and physiographic history of Pecos Valley in Texas
(University of Texas at Austin. Bureau of Economic Geology, 1962) Leonard, A. Byron (Arthur Byron), 1904-
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.