The Calloway Well Formation or Tuff is named from exposures in the Grant Range by Scott (1965).

The Calloway Well Tuff is Oligocene in age with K-Ar determinations on biotite of 33.2 +/- 0.7 Ma (Armstrong, 1970).

The Calloway Well Tuff is a composed of 1 to 9 crystal-rich rhyolitic ash-flow tuff cooling units with intervening layers of volcaniclastic sediments (Scott, 1965). These flows typically contain about 30 percent phenocrysts, 20 percent quartz, 20 percent sanidine, 10 percent plagioclase, and 10 percent mafics (Moores and others, 1968).

Thicknesses within the Calloway Well Tuff vary from 225 to 750 feet in two sections measured about 7 miles apart in the Grant Range (Scott, 1965).

The Calloway Well Tuff is only exposed within the Grant and Quinn Canyon Ranges (Kleinhampl and Ziony, 1985).