A. Byron Leonard

Correlation of Pleistocene Deposits of the Central Great Plains with the Glacial Section

Integration of Pleistocene chronologies of the central Great Plains and the glaciated area is a major problem of late Cenozoic stratigraphy in North America. Lenticular deposits of volcanic ash associated with fossil mollusks occur in both regions and furnish a widespread datum for interregional correlations. The ash lentils, collectively called Pearlette, can be differentiated petrographically from other late Cenozoic ash deposits of the Plains region and have been studied at localities extending from southeastern South Dakota to northwestern Texas. The associated molluscan fauna possesses an unforeseen degree of uniformity and stratigraphic significance. The Pearlette ash and faunal zone occurs above Kansas till and below Loveland loess and Iowa till in the Missouri Valley region and is judged to be early Yarmouthian in age. A modification of stratigraphic names for Kansas contributes to uniformity of terminology in the Plains region.

Stratigraphy of the Late Pleistocene Loesses of Kansas

The post-Yarmouthian Pleistocene of Kansas is characterized by extensive loess sheets which contain buried soils. The loesses and associated fluvial deposits are classed in Kansas as Crete, Loveland, Peoria, and Bignell members of the Sanborn formation. Based on lithologic characteristics, sequence, content of fossil mollusks, and stratigraphic continuity of buried soils, the members are identified throughout their extent in Kansas. Also trustworthy correlations with late Pleistocene deposits in Nebraska and Iowa, including the type Bignell and Loveland loesses, can be made. The Peoria loess of Kansas is shown by faunal evidence to be equivalent to the Peoria (Iowa and Tazewell) loess of central Illinois.

Ecological Conditions Accompanying Loess Deposition in the Great Plains Region of the United States

There has been lack of agreement concerning the ecological conditions of loess deposition in the central Great Plains region. Data from the molluscan faunas and the physical nature and distribution of the deposits are reviewed. They indicate that the surface of deposition was well vegetated with forest conditions along the eastern Missouri Valley and a grass cover on the western plains; the climate was somewhat more moist, and the temperature (although slightly cooler, on the average) had less severe extremes than at the present; and the primary source of the silt was in the outwash-carrying valleys during an episode of continental glaciation.