94 GLACIAL AXD SUEFACE GEOLOGY OF THE PACIFIC COAST. 



The Great Valley of California was once a lake of magnificent dimensions, 

 but it now retains its lacustrine character only at its extreme southern end. 

 The orographic depression between the Sierra Nevada and the Coast Ranges 

 must once have been very deep, as well as of great length, since it has 

 been the recipient of all the detrital material swept down from both sides, 

 although in vastly larger quantity from the eastern, during a long period of 

 time. How deep these accumulations are is unknown ; a bore of a thousand 

 feet in depth at Stockton, about midway in the valley longitudinally, did not 

 reach its bottom.* A large part of the southern end of the San Joaquin 

 Valley is liable to be converted into a lake after a succession of unusually 

 rainy seasons. It is a swampy region overgrown with tules, having one 

 large but verj' shallow permanent body of water near its centre, and about 

 midway between the extreme southern end of the valley and the point where 

 the San Joaquin River debouches from the Sierra. This lake, called Tulare, 

 from the growth of tules (Scirpus palustris) which surrounds it, has an area 

 at ordinary tiuies of 600 or 700 square miles, and a depth of only forty feet ; 

 but its dimensions vary considerably from 3'ear to 3ear. The reason why 

 water stands in the southern poi-tion of the San Joaquin Valley is, that the 

 rivers coihing down the slope of the Sierra farther north have filled up the 

 depression more rapidly than those to the south, and thus formed a kind of 

 dam or barrier to the escape of the water, or to a perfect drainage of the 

 southern part of the valley. This larger supply of detrital material from the 

 more northern streams is due in part to the increased precipitation as we go 

 north on the slope of the Sierra, in part to the greater development of the 

 belt of schistose and more easily disintegrated rocks, and also, to consider- 

 able extent, no doubt, to the vastly increased supply of volcanic material 

 north of the San Joaquin River, which in places forms deposits of great 

 thickness aloni;; the foot-hills, and much of which was of a nature to be most 

 easily and rapidly eroded away. 



The western slope of the Sierra Nevada is almost entirely destitute of 

 any lakes, except those very minute ones high up in the range, to be pres- 

 ently described. Bodies of water of considerable size once existed there, 

 although not in abundance ; but they have nearly all disappeared, having 

 become converted into sedge-grass meadows, or " flats," as they are usually 

 called. The slope of the range and its orographic simplicity are manifestly 

 unfavorable to the development of permanent lakes; the depressions which 



* For some details as to the form of the cross-section of the Great Valley, see Auriferous Gravels, p. 2. 



