It preserves fossils dating back to the Chesterian (Serpukhovian) Age of the Carboniferous period.

[1] At its type section in the Hueco Mountains, the formation consists of 30 meters (98 ft) of olive-gray shale, shaly sandstone, and dirty limestone.

[1] The formation is interpreted as having been deposited during a marine regression (withdrawal of the sea from the continent) to the south.

[2][1] These include abundant specimens of the ostracod Graphiadactyllis arkansana, as well as specimens of the bryozoan Archimedes terebriformis, the echinoderm Pentremites, the brachiopods Composita subquadrata, Spiriferina transversa, Diaphragmus elegans, and several species of Dictyoclostus,[2] and the trilobite Paladin helmensis.

[5] The original definition included the entire set of beds between Silurian and Pennsylvanian in age.