South Mountain Geology

From our Park Ranger Peter White, if you are looking for adventure and amazing geology just outside the city of Phoenix, then South Mountain Preserve is a must-see! The South Mountains are located just south of the fifth-largest city in the United States: Phoenix, AZ. Phoenix is about 20 kilometers away from the confluence of the Salt and Gila Rivers, the range has also been called the Salt River Mountains on some older maps. The range is 18 kilometers long with the elevation ranging from 1,200 – 2,600 feet. The highest point at South Mountain is Mt. Suppoa at 2,690 feet and, unfortunately, is not accessible to the public. While you are in the preserve, there are some well-known attractions you should stop at such as Dobbins lookout (2,330 feet, the highest point you can drive to), Pima Canyon (east), Telegraph Pass and the South Mountain Environmental Education Center.

The South Mountains consist of three separate ranges: Guadalupe (east), Gila (west) and Ma Ha Tauk (west). Although they are in the same area, the age difference between the eastern and western ranges varies greatly. The western half of the range is made up of metamorphic and granite rocks which are dark in color with light-colored bands, and were formed under high pressures and temperatures. These rocks can be further categorized into Estrella gneiss and Kamatke granite; they were formed in the Precambrian era and are about 1.7 billion years old. The eastern half of the range is made up of tertiary pluton which was formed by molten rocks solidified in chambers below the Earth’s surface. These rocks can be further categorized into Dobbins alaskite, Telegraph Pass granite and South Mountain granodiorite. These rocks were formed in the Cenozoic era and are only about 25 million years old. The eastern half is also covered in a dark-colored substance called desert varnish, which is a very small bacterium that takes upwards of 10,000 years to completely cover a rock surface. This is where you will see many petroglyphs carved into.

Whether you come out to South Mountain Preserve to hike, bike or take a drive to Dobbins Point, be sure to look at the difference between the eastern and western halves and where the dividing point is ((hint: Telegraph Pass). Which half looks more eroded? Is there a difference in the plant growth? What factors can cause erosion? Once you have your ideas, swing by the South Mountain Environmental Education Center and we’ll answer any other questions you might have.