1922] Vaughan: Geology of San Bernardino Mountains 



327 



The surface of the first cycle must have been developed before the 

 inauguration of the upward movement. The rolling topography 

 around Bear Valley represents a halt in this movement for a sufficient 

 time to reach late maturity. It is altogether probable that this con- 

 dition also existed in the Santa Ana. Renewal of the upward move- 

 ment would cause rejuvenation of the streams and deepening of the 

 canons. Bear Valley has a smaller catchment area than Santa Ana 

 Canon and is drained by a stream having a longer course and hence 

 requiring greater time to cut back to the valley. Hence the latter 

 might be greatly deepened and widened by erosion while Bear Valley 

 would change but little. A lowering of the rainfall in the region 

 would result in extensive deposition of fanglomerate both in the Santa 

 Ana and Bear Valley regions. 



It is well to point out here that in an arid or semi-arid region the 

 rainfall is somewhat dependent upon the relief. Examples are found 

 the world over where there is an abundance of rain in mountains 

 surrounded by desert. So here rains are frequent during July and 

 August in the mountains, while all around them there is no rain from 

 April to December. It is therefore difficult to say to what extent the 

 present rainfall is due to change in climate and to what extent it is 

 due to uplift since the deposition of the fanglomerate ; but certain it 

 is that we now find humid conditions modifying a topography devel- 

 oped under arid or semiarid conditions. 



It is thus seen that the topography of the Bear Valley and Santa 

 Ana areas is not the result of a simple erosional cycle, and that the 

 former was evolved at an earlier date than the latter. The third cycle 

 represented by Santa Ana Canon is found in many other places and 

 in some instances' is even more readily distinguished from the Bear 

 Valley or second cycle. In this connection the topography of the 

 south flank of the range will be described at length. 



Along the west side of San Gorgonio River a bench extends from 

 one-half to one and one-half miles wide, known as ' ' Banning Heights. ' ' 

 To the north and west rise hills of mature aspect. The slopes are 

 smooth and continuous with the heights. The graded condition of the 

 hills is indicated by the fact that, when the slope equilibrium is dis- 

 turbed by erosion at the bottom, small landslides are developed and the 

 whole hillside is quickly rejuvenated. The rocks from which these 

 hills are carved are granites and schists similar to those farther north, 

 but so badly decomposed that a pick can be readily driven into them 

 in many places. The present cycle is sufficiently developed around the 



