GEOLOGY OF CARRIZO MOUNTAIN, CALIFORNIA 351 



complete that the majority of these summits are reduced to mere 

 points and Hnes. In these cases the arroyos and the slopes below 

 the summits are often cumbered with the river wash which has 

 slumped down as the hills have been reduced, but has not yet been 

 removed. This material represents a variety of metamorphic and 

 igneous rocks probably derived from the Peninsula Range and its 

 outliers. Its deposition dates back to an earher erosional cycle, 

 when the present tops of the bad-land hills formed the bottom of 

 the valley of the ancestor of Carrizo Creek. 



Within Carrizo Valley proper and in the lowland south of Carrizo 

 Mountain, there is the usual desert accumulation of washed material 



Shale Sandstone Tuff Limestone with 



and conglomerate intrusive granites 



both mstamorphosed 



Fig. 9. — Diagrammatic section across Carrizo Mountain by way of Alverson and 

 Garnet Canyons. 



grading upward toward the mouth of each canyon into alluvial 

 cones. West of Carrizo Station and at other favorable points in 

 adjacent parts of the desert, there are many sand dunes, but these 

 reach a greater development in the more open desert to the north 

 and east. 



Passing eastward beyond the valley of Carrizo Creek toward 

 Imperial, the road at first traverses areas of well-reduced shales and 

 sandstones, and then reaches a broad zone of beach gravels full of 

 well-preserved shells of modern types. ^ This zone represents the 

 beach line of the extinct lake that once occupied the Colorado 

 Desert. The beach is not as distinct as a physical feature here as 

 at many points farther to the north (Fig. 8) because the shore line 



I Robt. E. C. Stearns, Ph.D. "The Fossil Freshwater Shells of the Colorado 

 Desert, Their Distribution, Environment, and Variation," Proc. U.S. Nat. Museum, 

 XXIV, 271-99. 



