Lenticular deposits of volcanic ash in the Miocene and Pliocene strata of western Kansas and western Nebraska are described petrographically. These include 14 ash falls of known stratigraphic position. All the ash studied is classed as vitric tuff and most of the falls are rhyolitic in composition. A regular or progressive change through time in the character of the ash does not occur. Ash falls are particularly common in the lower half of the Ash Hollow member of the Ogallala formation where it is estimated that volcanic material constitutes more than three percent of the volume of the sediments.