180 University of California Publications in Geology [Vol. 13 



Climate and vegetation. — In common with the greater part of Cali- 

 fornia, San Diego County has a summer dry season and a winter wet 

 season. In the mountainous portion of the county, however, there are 

 frequent local thunder showers during the summer. 



The Cuyamaca region receives probably more rainfall than any 

 other part of the county. At Cuyamaca Dam and at Pine Hills Hotel 

 the records show an annual precipitation of about 40 inches. A por- 

 tion of this is in the form of snow. In the winter of 1917-18 there 

 were 18 inches of snowfall at the dam, an amount said to be less than 

 usual. At the east end of the reservoir the average rainfall is not 

 much over 20 inches and farther east the precipitation rapidly de- 

 creases, the climate as a matter of fact becoming that of the desert. 



The vegetation of the region is an index of the distribution of the 

 rainfall. The Cuyamaca Peaks are covered with a dense growth of 

 pines, cedars, and chaparral. On the gentler slopes around Julian, 

 Pine Hills, Cuyamaca Reservoir, and Green V alley there is a more 

 open growth of pines, with oaks and in places dense stands of chapar- 

 ral. At the east end of the reservoir the vegetation is scanty and the 

 divide between the Pacific and desert drainages in certain stretches 

 is almost barren, in other stretches it has a chaparral cover with scat- 

 tered trees. To the east of Banner and Chariot caiions the country 

 gradually takes on the appearance of the desert, many of the rugged 

 slopes being almost barren of vegetation. 



Physiographic problems. — The Cuyamaca region is but a small 

 part of a large physiographic province about which little is known. 

 It is impossible to work out the physiographic history of such a small 

 area without extensive observations over the larger area. Lacking 

 this knowledge of the region as a whole, it is thought best not to pre- 

 sent any conclusions based on the meager data at hand. Attention is 

 called, however, to certain physiographic features which will need 

 explanation in any future investigation of the physiography of the 

 region. These features are : 



(1) The alluviated summit valleys; (2) the attack of the streams 

 on the alluvial filling of these valleys ; ( 3 ) highest mountains situated 

 to the west of the main water parting; (4) highest mountains com- 

 posed of rocks yielding easily to chemical agencies ; lower country 

 carved from formations resistant to chemical decay; (5) the deep 

 canon of Boulder Creek cut through the highest mountain range ; 

 (6) the unimportant elevation of the main water parting above the 

 adjacent summit valleys; (7) contrast in physiographic features on 

 either side of the main divide. 



