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.