The section of New Mexico Highway 15 between the intersection of NM-15 and NM 35 (aka Sapillo junction) at the south and the Gila Cliff Dwellings National Monument at the north end of NM –15 occupies an approximately 18 mile long, mile wide, corridor through the eastern part of the Gila Wilderness (Fig. 1). Whereas most of the Gila Wilderness is dominated by silicic, caldera-forming supervolcanoes of Eocene to Oligocene age, this part of NM-15 traverses a volcanic terrain of similar age, but composed mainly of intermediate composition lava flows and minor associated rhyolitic intrusions and pyroclastic rocks, which are related to the here-named Copperas Creek volcano. This volcanic complex is bounded by Basin and Range structures: on the south by the Sapillo Creek graben, and on the north by the Gila Hot Springs graben, both of which are filled with Gila Conglomerate of late Tertiary to Pleistocene(?) age. Hot springs in the Gila River valley are localized along faults in the deepest part of the Gila Hot Springs graben. The cliff dwellings of the National Monument were constructed in caves in Gila Conglomerate in the western part of the Gila Hot Springs graben. The eastern edge of the Gila Cliff Dwellings caldera is buried by younger rocks east of the cliff dwellings, but spectacular cliffs of Bloodgood Canyon Tuff, which fills the caldera, can be viewed along the West Fork of the Gila River from the trail starting at the cliff dwellings. Although this is not intended as a formal road log, highway mileage markers (MM) will be used to locate geologic features more or less progressively from south to north along NM-15.
