First used. Exposed in floor of Lamar valley for 8 mi above Soda Butte Creek, northern Yellowstone National Park, WY in the Yellowstone province. Intent to name not stated. No type locality designated. Forms a 2,000 ft scarp. Lower half is early basic breccias, and upper half is basalt topped by a thin veneer of rhyolite on the west side of Lamar River. Rhyolite rests on breccias on east side of Lamar River. [Not clear as to whether the Lamar basalt, the formation, includes the underlying breccia and/or the overlying rhyolite.] The basalt is uniform in thickness. Of Miocene-Pliocene age. Geologic map.

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