About Basin and Range
The Basin and Range topography of southern Arizona reaches its climax in the Sky Islands region in the southeastern corner of the state. Here several mountain ranges rise more than 6000 feet above the surrounding basins, with Mt.Graham in the Pinaleno Mountains rising nearly 8000 feet. Though still considered part of the Basin and Range, the geology of the Sky Island region is quite complex and not easily categorized as such. A great variety of rock types and geologic features are found in the Sky Island region. Sedimentary, metamorphic, and igneous (volcanic and plutonic) rocks are exposed in several different areas. The Rincon, Santa Catalina, and Pinaleno Mountains are considered by geologists to be metamorphic core complexes. These mountains are "cored" with a mixture of igneous and metamorphic rock, some of which has been highly deformed. The "inside" of these mountains has been revealed as faulting uplifted the ranges and erosion stripped away the overlying rocks, exposing their core.
A period of intense volcanism, beginning about 25 million years ago and continuing for several million years, occurred in the region depositing large amounts of volcanic ash and lava across the landscape. A volcanic rock called tuff, which is essentially a solidified hot ash flow, formed during some of these eruptions. The fantastic rock spires and hoodoos of Chiricahua National Monument in the Chiracahua Mountains formed from the erosion of this welded tuff. http://dana.ucc.nau.edu/~are-p/road_map/eco/geology.html