Geology from a Motor Car 351 



of the canyon is 6,050 feet, and descends to 2,650 feet at the 

 junction with Cajon Canyon. Lone Pine and Cajon canyons 

 join near Cosy Dell. From Cosy Dell the highway leads up 

 Cajon Canyon over the summit or rim of the mountains to the 

 broad arid plain of Mojave Desert, to Victorville. 



Upper Mojave Valley Superimposed 

 upon the Plain 



The upper Mojave Valley is a great alluvial plain that slopes 

 gently northward from the San Bernardino and San Gabriel 

 Mountains. The plain has been formed from disintegrated rock 

 debris washed down from these and other mountains. The 

 upper Mojave River flows through a channel eroded in the deep 

 alluvial soil. It is somewhat of a surprise to find that the river 

 has cut through hard granitic rocks at the Narrows, north and 

 south of Victorville, instead of going around them. The river 

 is what geologists call a superimposed stream, that is, one let 

 down upon the landscape regardless of the character of the 

 rocks. The river formerly flowed at a higher level, its chan- 

 nel cut in the soft alluvial soil of the plains. As the plain was 

 lowered by erosion the river cut down its channel, and when 

 the hard granitic rocks were encountered, its channel having 

 been established, the river cut down into the rocks as fast as 

 the land rose. Hence the river now follows its original course 

 through the rocks rather than around them. 



Desert Plain Broken by Many Moiintains 



The surface of the Mojave desert plain is broken by many 

 mountains, hills, and ridges. Loosely consolidated alluvium is 

 widespread over the valleys, "wash" from the mountains and 

 hills. Sheet erosion the action of wind, rain and heat under 

 arid conditions results in the slow crumbling of the rocks and 

 the accumulation of gravel, sand, and clay in the valleys. The 

 vast plain of the desert stretches far to the north and east. 

 Granitic mountains rise above the plain. Alluvial wash, gravel, 



