262 Adventures in Scenery 



fill," as distinguished from the "older alluvium" represented by 

 the clays, shales, and sands of the earlier Fernando epoch. 



The Great Valley a Faulted Sunken 

 Basin 



South of the foot of the great escarpment that forms the 

 south wall of the San Bernardino and San Gabriel ranges lies 

 The Great Valley of southern California. Its depressed faulted 

 bottom is deep below the present surface, and the nature of the 

 bed-rock floor is not known, as it has not been reached by the 

 deepest wells. It is probably 1,000 feet or more below the 

 present surface of the alluvial valley fill. East of the Bunker 

 Hill Dike, east of the washes of Lytle Creek and El Cajon Can- 

 yon, the valley is known by the name of the mountain range 

 that overhangs, San Bernardino. To the west the valley is 

 known as the San Gabriel Valley. It lies on the depressed floor 

 below the wall of the uplifted San Gabriel Mountain range. 

 The San Gabriel Valley extends westward as the La Canada 

 Valley, the bottom of which is a depressed fault block which 

 cuts off the San Rafael Hills and Verdugo Mountains from the 

 San Gabriel Range. 



Throughout its whole length this sunken valley floor is 

 buried beneath the porous sediments which have been carried 

 down from the high mountains through canyons that have been 

 cut by torrential streams which descend the steep faulted 

 scarps of the mountain ranges. At the mouth of each canyon 

 the stream builds a cone or alluvial fan from the detritus eroded 

 from the rocks of the high land. The cones or fans are built 

 up of layer upon layer of rock fragments of all conceivable 

 sizes from boulders weighing many tons to the finest sand, silt, 

 and clay. Naturally the coarser materials and heavy blocks of 

 rock are thrown down in the mouth of the canyon at the apex 

 of the cone or apron. Succeeding storms may and do result in 

 unusually powerful torrents of water, and these may and do 

 pick up and move further down the slope of the cone or fan 



