The Amsden formation was described by Darton1 in 1904 from its outcrops along the Amsden River on the east slope of the Big Horn Mountains in northern Wyoming. Since then he has described and mapped it along the Wind River Mountains from Circle on the north to south of Dallas on the south, in the Rattlesnake Mountains, and in the Owl Creek Mountains, and Blackwelder writes of its occurrence in southern Montana. According to Blackwelder, “it can be followed with more or less confidence clear across the State (Wyoming) from the Black Hills to Idaho.” 2 Only the Amsden of the east slope of the Wind River Mountains from Bull Lake southward is treated in this paper.
Darton has called the upper Amsden Pennsylvanian, on the basis of fossils collected from near Leo and Shirley, and the lower part probably Mississippian. Blackwelder3 has found two faunas . . .