334 University of California Publications in Geology [Vol. 13 



The streams south and east of Rattlesnake flow in newly cut chan- 

 nels in their lower courses. This may be seen between Rock Corral 

 and Rattlesnake. On the plateau the stream courses are so crooked 

 and the grade so low that one hardly knows whether he is going up 

 stream or down ; but on the north side of the range they drop through 

 steep juvenile canons. 



All the erosion going on at present cannot be considered as due to 

 streams. Between Rock Corral and The Pipes typical desert erosion 

 is active. The low granite ridges of this area are burying themselves 

 in their own detritus except where the water is able to carry it away. 

 There is no true exfoliation, i.e., no curved fragments are being formed. 

 The detritus consists almost wholly of disintegrated granite with occa- 

 sional angular or subangular harder fragments of pegmatite and aplite. 



The streams along the south flank of the range flow in youthful 

 canons cut far below the mature surface of the third cycle, some parts 

 of which form prominent benches above the streams. Such is the case 

 at Banning Heights, which rise above the San Gorgonio Pass on the 

 south and above the San Gorgonio River on the east. Morongo Valley 

 opens to the east and was evidently excavated by drainage in that 

 direction, but it has been captured by streams — Dry, Big, and Little 

 Morongo creeks — cutting back from the south. 



The youthful topography may also be seen in the higher parts of 

 the mountains. In Santa Ana Canon the general appearance of ma- 

 turity is preserved, but in reality it is deeply incised by erosion, which 

 is still active (pi. 20A). The main stream has cut into the fanglom- 

 erate to a depth of nearly five hundred feet. In Mill Creek Canon 

 erosion has gone farther and has almost completely removed the old 

 valley floor. The smooth ridge just south of Raywood Flat is incised 

 by V-shaped gullies (pi. 21B). Toward the east both the ridge and flat 

 are giving way to a youthful rugged topography as the streams cut 

 back. This same sort of thing may be seen in nearly all the canons 

 immediately to the north. 



At the mouth of Mission Creek there are two terraces of fanglom- 

 erate between the present stream bed and the old surface on top of 

 the ridge to the west. In all the canons along the south, terraces are 

 found ten to twenty feet above the stream beds. These probably 

 represent changes in the lramidity of the region, although those at the 

 mouth of Mission Creek may possibly indicate halts in the upward 

 movement of the mountain mass. 



