Collier Shale

The Collier Shale is a geologic formation in the Ouachita Mountains of Arkansas and Oklahoma. Dating from the Late Cambrian to Early Ordovician periods, the Collier Shale is the oldest stratigraphic unit exposed in Arkansas. First described in 1892, this unit was not named until 1909 by Albert Homer Purdue in his study of the Ouachita Mountains of Arkansas. Purdue assigned the type locality to the headwaters of Collier Creek in Montgomery County, Arkansas, but did not designate a stratotype. As of 2017, a reference section for this unit has yet to be designated.