The first recorded evidence of beds of pre-Mississippian age in Kansas was in 1908. These range in age from pre-Cambrian to Devonian, but do not crop out in the state. Wells drilled for oil since 1925 have furnished much information regarding the character and extent of these older Paleozoic rocks. Their present areal distribution is the result of a long history of repeated earth movements of regional extent. The two most important movements occurred between Devonian and Mississippian and in post-Morrow time. The duration of erosion periods was progressively greater toward the northwest.

Although the paper principally concerns Kansas, where the data have been accumulating for 4 years, an effort has been made to present new data in Oklahoma and bring the map up to date. The map represents the areal geology of most of Kansas and Oklahoma as it would appear if all the sediments were removed down to the base of the Mississippian.