Rifle Gap lies on the edge of the White River Plateau, just north of the Grand Hogback. The Grand Hogback monocline consists of Upper Cretaceous Williams Fork Formation sandstone, shale and coal beds that are approximately 4,500 feet thick. From the reservoir, views to the south offer unparalleled exposures of the coal-bearing Mesa Verde group in the hogback. Most of the park is comprised of exposed Mancos Shale, and as a result, park soils are rich in salts. Fossils of leaves are abundant, and some fish and shellfish fossils exist as well.