It is generally faulted along its western side, and varies in height, with gaps in numerous locations where rivers exit the mountains.

The hogback was formed during the Laramide orogeny, approximately 50 million years (50 my) ago, when the modern Rockies were created.

The general uplift to the west created long faulting in the North American Plate, resulting in the creation of the hogback.

North of Denver its major gaps are I-80 in southern Wyoming, U.S. Highway 34 at Loveland, and U.S. 36 to Rocky Mountain National Park.

Thus, oyster and clam shells, sand, and mud built slowly into layers of sandstone, shale, limestone, and "mudstone.