Three Twins member of Chalk Bluff formation. North from Little McKittrick to Spencer Draw the top of the Carlsbad limestone grades into sandstones, which in turn are in part displaced by anhydritic sands, sandy red beds, anhydrites, fine greenish sandstones, thin greenish-gray sandy shales and dolomitic limestones. This sequence of beds is here named Three Twins member of Chalk Bluff formation, from Three Twins Hills, Pecos Valley, Eddy Co., southeastern NM, where, in Spencer Draw, a partial section is exposed. This member is top part of the formation. It is underlain by Azotea tongue of Carlsbad limestone, or, where that is absent, by lithologically similar Seven Rivers member. The Three Twins and Seven Rivers members represent greater part of Chalk Bluff formation and are = Carlsbad and Capitan limestones of reef zone. Age is Permian.

Source: US geologic names lexicon (USGS Bull. 896, p. 2144).