A glacial till underlies the Bishop tuff, but it has not been established heretofore on firm geological bases that this is the Sherwin till. Relationships here described demonstrate that the till beneath the Bishop tuff is indeed Blackwelder's-type Sherwin.
Radiometric datings of the basal pumice of the Bishop tuff by Dalrymple and others yield an age of 710,000 years. Judging from pre-pumice weathering, the till may be still older, perhaps 750,000 years. These considerations make the Sherwin till-Bishop tuff relationship an important reference datum in the continental Pleistocene sequence of North America.