Named as upper member of Spears Formation of Datil Group. Type section measured approximately 14 mi northeast of Datil and 1.25 mi north-northwest of Rincon Windmill, NW1/4 NW1/4 sec 18, T1N, R8W, northeast Datil Mountains, Socorro Co, NM, Orogrande basin. Also known in Gallinas Mountains. Composed of volcaniclastic sandstone, brown to light brown, gray, very coarse to fine- to medium- to granule sand with pebbles and cobbles, moderately to well sorted but very porous, well stratified, some low-angle cross-bedding and some planar bedding. Some beds have abundant granules of pumice at type where member is 411 ft thick and where it overlies Rock House Canyon Tuff (new) and underlies Hells Mesa Tuff (new). Interbedded with Blue Canyon Tuff. Ranges from 275-500 ft thick. Not dated. Assigned to Oligocene.

Source: GNU records (USGS DDS-6; Denver GNULEX).