The valley of the North Fork of the Shoshone River passes through a series of volcanic rocks over 9,000 feet thick covering 3,000 square miles. The rocks include lava, volcanic ash, and other sorts of volcanic material. Agglomerate is a common type and consists of rounded masses of volcanic rock in a finer matrix. Numerous dikes which were feeders for lava flows, show in the canyon walls as thin, narrow bands resembling stone walls. The peculiar castle-like forms are the result of weathering and removal of softer material by water.