Its age is poorly constrained but is thought to be Namurian (late Mississippian to middle Pennsylvanian).

This is overlain by an informal middle member, consisting of mudrock, sandstone, and conglomerate, that is typically around 40 feet (12 m).

The upper member is typically around 25 feet (7.6 m) that includes maroon to light gray sandstone.

[1] The Coalbank Hill Member may be a terra rossa (a residual soil formed by weathering of the underlying Leadville Limestone) and may correlate with the Log Springs Formation in the Jemez Mountains of New Mexico.

The formation was first designated by Charles Whitman Cross and Ernest Howe in 1905 for deposits near Molas Lake.