First used as map unit at Pilot Knob, a hill west of the community of Pilot Knob, Travis Co, TX south of Austin, TX in the Ouachita tectonic belt province. Type locality not designated, and derivation of name not given. Intruded rocks of the Austin Group. Is dark-greenish-black to black, microcrystalline basalt that weathers to a brown or rusty color. Is only known to be exposed in the vicinity of Pilot Knob where it makes up the core of Pilot Knob itself and a number of small surrounding hills. [Stratigraphic relations are inconsistently presented. Geologic map explanation (pl. VII) lists the basalt below both the Austin Group and the Pilot Knob tuff. However, Table 7 shows the basalt as a formation at top of Austin Group, above Pilot Knob tuff.] Age is Late Cretaceous.

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