Biotite-bearing chalky-white rhyolitic ash, here called the Bishop ash bed, occurs in-middle Pleistocene alluvial and lacustrine deposits at eight localities scattered from California to Nebraska and is correlated with the basal air-fall lapilli of the Bishop Tuff, an ash flow of eastern California, radiometrically dated about 0.7 million years. Correlation of the Bishop ash bed with the air-fall lapilli is made on the basis of similar petrography and on chemistry as determined by electron microprobe, atomic absorption, and emission spectrographic analyses. At five localities the Bishop ash bed lies stratigraphically below a Pearlette-like ash. As more occurrences of the Bishop ash bed are found, it should become an increasingly important dated stratigraphic marker relating middle Pleistocene deposits and events across several geomorphic provinces.
Another biotite-bearing chalky-white ash, here called the ash of Green Mountain Reservoir, occurs at three other localities and is distinguishable from the Bishop ash bed by small differences in chemical composition of the glass. The ash of Green Mountain Reservoir is younger than the Bishop ash bed, as shown by the fact that at one locality it lies stratigraphically above the aforementioned bed of Pearlette-like ash.