A reconnaissance of Pliocene and Pleistocene geology, and Pleistocene fossil mollusks was made for the region of east-central New Mexico, including the High Plains and Pecos valley areas, northward from northern Eddy County to the south side of Canadian River valley. The Ogallala Formation (Pliocene) was deposited by streams flowing east-southeast, resulting in a broad alluviated plain that masked the earlier erosional topography. Early Pleistocene time witnessed structural tilting and warping, increased precipitation, and development of a new drainage system that flowed eastward through the now abandoned Portales valley into Texas. An independent southward drainage system captured the drainage through Portales valley in the vicinity of Fort Sumner, and thus produced the present Pecos River.Molluscan faunal assemblages from 45 localities produced a composite fauna of 47 species; 2 assemblages are from deposits judged to be Kansan, 7 assemblages are from Holocene terrace deposits dated at 6,000 to 5,000 radiocarbon years B.P.; the remaining assemblages are Woodfordian, 7 of them radiometrically dated at ages ranging from 18,000 to 13,000 B.P. Inferences drawn from the molluscan faunas include: 1) the 2 Kansas faunas lived in a xeric environment not unlike that produced by the present-day climate; 2) a long, perhaps intermittent pluvial period existed in the region from 18,000 to 13,000 B.P. and perhaps longer; and 3) by about 5,000 B.P. the present semiarid climate was well established. While the long pluvial period produced environments suitable for branchiate gastropods, many aquatic pulmonates, and a variety of terrestrial gastropods, evidence of forest cover or a climate significantly cooler than today is not clear.

Two outstandingly productive sections in northeastern New Mexico yielded fossil seed floras that confirm the correlation between clay-mineral zonation in the Ogallala Formation and that based on fossil floras. The fossil seed floras place the sections in the Ash Hollow and lower Kimball members. The Kimball Clayton South section is remarkable for the substantial seed flora it contains and is unique for the well-preserved molluscan fauna of 26 taxa, three of which have been described as new. The Seneca Northeast section not only contains a distinctive seed flora but is outstanding for the occurrence of three species of hackberry: Celtis hatcheris Chaney, hitherto known only from Oligocene sediments; C. willistoni (Cockerell) Berry, the common and widespread hackberry in the Ogallala Formation; and C. cf. reticulata Torrey, a species living in the area today. Ecological implications of the Clayton South molluscan fauna indicate no remarkable difference from present climate, except for lesser temperature extremes and rainfall sufficient to maintain natural marshes and/or (possibly ephemeral) ponds.