The early Oligocene Copperas Creek volcano and geology along New Mexico Highway 15 between Sapillo Creek and the Gila Cliff Dwellings National Monument, Grant and Catron counties, New Mexico
— James C. Ratte

New Mexico Highway 15 between Sapillo Creek and the Gila Cliff Dwellings National Monument provides a tour through the eroded remains of the ~ 30 million year old Copperas Creek volcano, as preserved between the west-northwest -trending Sapillo Creek and Gila Hot Springs grabens of Basin and Range age. Colorful exposures of altered volcanic rocks in road cuts and a scenic overlook of the Alum Mountain eruptive center are witness to the hydrothermal alteration and mineralization in a Yellowstone-type hot spring environment here in Oligocene time. New Mexico Highway 15 ends at the Gila Cliff Dwellings where alcoves in Gila Conglomerate were occupied by members of the Mogollon culture 700-800 years ago.