Two and a half billion years of geologic history are exposed in the canyon walls of Shell Creek. The gradual cutting action of Shell Creek eventually exposed the basement granite layer that lies above the earth’s molten core. Shell Canyon itself formed during the past million years and is considered very young by geologic standards, with a V-shaped profile and narrow canyon walls. The canyon began when a fault split the granite bedrock, and water followed the path of least resistance in a continuous downward cutting action that exposed many distinct rock layers. Madison Limestone and Bighorn Dolomite, both formed under warm, shallow seas, are characterized by shell fossils. Basement rock, also called bedrock, is the oldest layer lying closest to earth’s molten core.