The Coachella Graben

Note:  The following data was inspired by James Cornet’s article, “Mountins continue to rise”, in the Desert Sun on Sunday, January 8, 2006.  James Cornet is a desert biologist living in Palm Springs. E-mail him at jcdesertun@aol.com

 

Coachella Valley is not a true valley, it is a Graben.  True valleys are created by water eroding the soil and rock over millions of years.  However, the Coachella Valley is what geologists call a graben.

 

 

In the case of the Coachella Valley, the fault is the San Andreas Fault and its associated faults.  It is doubtful the name Coachella Graben would have been acceptable to the first settlers.

There are five mountains ranges around the valley.  To the east of Mecca lie the Orocopia Mountains; at just 3,815 feet they are the lowest range. 

The northern boundary of the Graben is formed by the Little San Bernardino Mountains.  Eureka Peak at 5,518 feet is located in Joshua Tree National Park.

On the northwest is San Bernardino Mountains with San Gorgonio Peak at 11,499 feet, it is the highest peak in Southern California.

The above three mountain ranges form what geologists call the Transverse Ranges Geological Province. Only the northern most 120 miles of the 900 mile range lies within the United States. Most mountain ranges in North America are aligned north south; the Peninsula Transverse Ranges are aligned east west.  This configuration results from the San Andreas Fault; the meeting point between the Pacific Plate and the North American Plate.  This process started some 25 million years ago with the formation of the San Andreas Fault.

The southern edge of the Coachella Valley is bound by the Santa Rosa Mountains.  Toro Peak is the highest point at 8,808 feet.  The Santa Rosa Mountains and the San Jacinto Mountains which are on the western edge of the valley lie within the Santa Rosa and San Jacinto Mountains National Monument.  The rock mass of the San Jacinto Mountains is classified as Mesozoic granite called batholithic granite.  From the valley floor these two mountains dominate the view by their ragged and steep shape.  The San Jacinto range rises from 800 feet to 10,804 feet in less than seven horizontal miles, creating the steepest escarpment in North America.

 

 

Definitions:

Batholith:

A large mass of subsurface intrusive igneous rock that has its origins from mantle magma.

Intrusive Igneous Rock:

A mass of igneous rock that forms when magma from the mantle migrates upward and cools and crystallizes near, but not at, the Earth's surface. Also called plutonic igneous rock. Also see dyke, sill, and batholith.