Benton Group

Older – Dakota
YOUNGER – NIOBRARA

Benton Group 100– 88 Ma: The small, flat valley between the Niobrara Formation and the towering red sandstone hogbacks was formed by the easily eroded Benton Group. Consisting mostly of black shales and limestones, this formation also contains thin lenses of volcanic ash clays.

These layers were created as the Cretaceous Seaway was encroaching.