265 



very large boulders as well as gravel and sand. Thus the cones 

 tend to flatten out apron-like farther from the mouths of the 

 canyons. When it is considered that the carrying power of a 

 stream increases as the sixth power of its velocity it will be 

 readily seen how boulders of tremendous size may be moved by 

 running water. Doubling the velocity means increasing the 

 carrying power 64 times. The velocity of a mountain stream 

 may be easily imagined to be three times as great following a 

 storm as during ordinary time. This would mean increasing 

 the carrying power more than 240 times. If a stream having 

 a velocity sufficient to move a rock weighing one pound were 

 doubled twice, that is, its velocity increased four times (which 

 could easily be imagined in mountain torrents) it would be able 

 to move a boulder weighing more than two tons. 



Alluvial Fans or "Washes" Formed 

 in Valley 



During the long time since the mountains were uplifted 

 streams have been cutting canyons and building cones or fans 

 at their mouths on the floor of the great depressed valley. Fans 

 formed by the great series of mountain streams have spread and 

 merged together so that a continuous floor of porous valley fill 

 or alluvium now' extends from the east end of San Bernardino 

 Valley to the valley of La Canada between the Verdugo Moun- 

 tains and the San Gabriel Range, and westward to the San Fer- 

 nando Valley (another faulted basin). 



The San Gabriel River descends from the mountains 

 through a deep canyon and crosses the San Gabriel Valley. It 

 has not merely built a cone or fan of boulders, gravel, sand, and 

 clay but it has spread a vast field of sand and gravel far to the 

 southward, known as the San Gabriel Wash. In a similar man- 

 ner San Dimas Creek debouches at the mouth of its canyon 

 upon the San Gabriel Valley at its narrowest point north of the 

 San Jose Hills, and has spread a vast fan or "wash" south and 

 west to Vineland, and its waters in flood seasons cross the porous 



