Exposed rock surfaces in the Pima Canyon Wash, South Mountain Preserve, Phoenix, Arizona. Please click any photo in the set for enlarged views.
“The eastern half of the South Mountains contains granitic rocks that are salt-and-pepper colored on freshly broken surfaces, but are generally coated by a brown or tan veneer of desert varnish. These rocks were formed in the Cenozoic Era only 25 million years ago, when molten rocks solidified in a large chamber several miles below the earth’s surface. The granites are the same age as volcanic rocks of similar composition that dominate the landscape of the Superstition Mountains east of Phoenix. While the granites were cooling and crystallizing, they were sheared by movement on a major, gently inclined fault zone that was discovered during geologic mapping of the range. The shearing took place at high enough temperatures, perhaps 3000 to 4000 C, to produce a foliation, or planar orientation of minerals, in granitic rocks on top of the range. As the granitic rocks continued to cool, the shearing became increasingly brittle and formed a breccia, a rock composed of angular fragments. The fault responsible for this shearing is no longer active and is not likely to cause earthquakes, but is probably present at depth below most of Phoenix, Tempe, and Mesa. Movement on the fault is responsible for the southwest tilt of the Cenozoic volcanic and sedimentary rocks of Camelback Mountain, Tempe Buttes, and Papago Park.”
- Excerpt from “Geologic History of the South Mountains” by Stephen J. Reynolds in Arizona Bureau of Geology and Mineral Technology Fieldnotes, Vol 15, No. 1, Spring 1985.