Over 30,000 feet of Lower Cambrian through Upper Pennsylvania rocks are exposed in the southern Egan and Schell Creek ranges. Approximate thicknesses by system are: Cambrian, over 13,000 feet; Ordovician, 4400 feet; Silurian, 1000 feet; Devonian, 5300 feet; Mississippian, 2100 feet; Pennsylvanian, 3500 feet. The Paleozoic has been divided into 24 formations, five of which are herein given new names. A Late Devonian unconformity progressively onlaps Upper Devonian beds in the northern and western parts of the area studied. Permian rocks are not present.

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