It preserves fossils dating back to the Famennian Age of the late Devonian period.

[3] The Ready Pay Member (formerly lower Percha) is mostly black fissile shale nearly devoid of fossils and with a total thickness of about 132 feet (40 m).

[1] The Box Member (formerly upper Percha), which is much less limited in areal extent,[3] is about 47 feet (14 m) of gray to green calcareous shale with limestone nodules and beds.

[1] The base of the formation contains fossils of arthrodiran fish, shark teeth, late Fammenian conodonts, brachiopods, and corals.

The Box Member contains fossils of brachiopods, crinoids, bryozoans, sponges, corals, and late Fammenian conodonts.