The Kishenehn Formation, a sequence of over 11,000 ft (3,350 m) of synorogenic deposits, has yielded fossils that include microfloral and megafloral remains, mollusks, insects, fishes, and mammals that suggest an age range between late Eocene and early Miocene. Radiometric dates from the deposits in the Kishenehn and South Fork basins are within the Oligocene. The fossils provide evidence for a climatic transition from sub-tropical to warm temperate conditions and development of several new taxa.