A number of diabase dikes and sills intrude the lower part of this member.

[2] The formation is thought to have been deposited in a subsiding marine basin (Bisbee basin[3]), which was subsequently filled in by delta deposits and capped by subaerial silica-poor (mafic) lava flows.

The fine-grained member is sparsely fossiliferous, containing shell fragments and forams that have been recrystallized to coarse-grained calcite.

[6] Age-diagnostic fossils are present near the base of the upper conglomerate member, including the bivalve mollusc Gryphaea mexicana, gastropods (Nerineidae) and the coral Thamnasteria.

[5] The formation was first defined by S.G. Lasky in 1938 for exposures at Broken Jug Pass in the Little Hatchet Mountains of New Mexico.