112 Adventures in Scenery 



Jacinto, Santa Rosa and other mountains of the Peninsular 

 Range, including the Cocopah (which is in Mexico) on the west 

 side of the desert, are ancient mountains. They consist of 

 granite, gneiss, and schists, igneous and metamorphic. They 

 are probably pre-Cambrian in age. They have been weathered 

 and eroded during the ages. Alluvial debris has been washed 

 down their slopes and forms deposits around the edges of the 

 desert basin. 



The Geologic Formations 



Formations of Tertiary age flank the mountain ranges, 

 known as the Mud Hills formation. Extensive beds of oyster 

 shells and other marine fossils occur on the west side of the des- 

 ert, notably along Carrizo Creek and at Coyote Wells west of 

 El Centre. Near Mecca, on the east side of the Salton Sea, the 

 basal deposit of the Mud Hills formation is a conglomerate. 

 This is a continental or land deposit as distinguished from a 

 marine or sea deposit. It is thought that this deposit was made 

 as the sea retired from the trough. The deposit of alluvial 

 strata of the Mud Hills formation was interrupted by violent 

 upheaval in late Tertiary or early Pleistocene time. The Mud 

 Hills formation is much bent, broken and contorted. Border- 

 ing the ranges of mountains that form the enclosing walls of 

 the basin are foothills of much disturbed Tertiary strata, rem- 

 nants of former alluvial aprons. Below these foothills are 

 deposits of recent and modern alluvial fill. The great struc- 

 tural trough itself is deeply filled with erosional debris from 

 the granitic, gneissic, and schistose rocks of the surrounding 

 mountains. 



Mountains of the Colorado and Mojave deserts are thought 

 to belong to one great mountain system. The mountains were 

 long eroded, and finally the whole region subsided. The pres- 

 ent mountain peaks and ranges stand out in a sea of erosional 

 debris washed from the slopes of these mountains and deposited 

 in the intervening valleys. These valleys are now broad flat 



