Mountains and Balanced Boulders

• The mountains of the Sonoran Desert are made of various rock types including granite, sandstone and volcanic ash. As the rocks weather away in the sun, wind and rain, many form balanced boulders. These boulders, if undisturbed by earthquakes or other forces, can become domes. These dome structures can be seen in many places around the desert including Camelback Mountain in Phoenix and around the high ridges on the west side of the Santa Catalina Mountains.
  
  

Alluvial Fans and Bajadas

• During rare heavy rains, the mountains experience mudslides full of rocks and vegetation that flow down through narrow canyons. When a mudslide reaches an area where it can spread out, it creates an alluvial fan that contains debris from the mountain. The areas where visible alluvial fans from different sections of the mountain meet are called bajadas.
  
  

Valleys, Streams and Flood Plains

• One or more stream beds typically run through each of the desert valleys. The flood plain surrounding each stream is also visible. Since the 1890s, floods no longer flow onto the flood plains. The streams now eat away at the bank, enlarging the stream bed. This is a result of number of factors including cattle grazing, the mining industry removing vegetation from the area and subtle climate changes.
  
  

Desert Pavement

• Desert pavement occurs in the hottest and most arid portions of the Sonora. This soil type occurs over a large, flat section of desert that contains little or no vegetation. One of the longest stretches of desert pavement in the Sonora is in the low mountains of the lower Colorado River Valley. The rocks are covered with what is called desert varnish, which gives the pavement a shiny black surface on the exposed rocks.
  
  

Sand Dunes

• According to the Arid Lands Information Center website, the Sonoran Desert has one of the most extensive sand dune systems in North America. These dunes are found in downwind sections of the desert where wind blown sand has accumulated. The sand for the dunes comes from various regions. For example, the Gran Desierto, Algodones dune and Mohawk Valley receive large quantities of sand from the Colorado River delta. The Cactus Plain receives its sand from the old shoreline of the Colorado River.