3 8o 



University of California. 



[Vol. i. 



tions of the valley, and it seems impossible there to draw any natural 

 line between the early Pleistocene and the alluvium, as the terms 

 are used in the U. S. geological folios. For the purpose of this paper 

 such distinction is not essential, and the term Pleistocene will be 

 used to include all the unconsolidated deposits of presumably post- 

 Tertiary age. 



The San Joaquin Valley. — This valley, which has been described 

 as draining northward through the San Joaquin River, resembles 

 in a general way the Sacramento Valley, with which, it is in direct 

 continuation. Our information in regard to its western and eastern 

 borders is, however, very meager. On the east, the low foot-hills 

 are composed of flat-lying Tertiary sandstones, fine tuffs and clays, 

 and gypseous beds, which rise very gradually out of the alluvium 

 and lap up over the older truncated rocks of the Sierra Nevada in 

 the long even slope already described. On the west, the Tertiary 

 and Cretaceous sandstones and shales have been highly folded * and 

 the slopes of the Coast Ranges emerge with comparative abruptness 

 from the plain. 



The general plan of the drainage resembles that of the Sacra- 

 mento Valley, but the southerly deflection of the streams flowing 

 down from the Sierra Nevada is not so great ; in fact, as would be 

 expected, they show slight northerly deflections when referred to 

 lines perpendicular to the general northwest and southeast trend of 

 the valley. Owing to the arid character of the eastern slope of the 

 Coast Ranges, the predominance in volume of the drainage from 

 the east over that from the west is much more marked than in the 

 case of the Northern Valley, the western tributaries of the San Joa- 

 quin River being quite insignificant and practically lacking in its 

 upper reaches. It is probably in consequence of this that the San 

 Joaquin River, in its course along the valley, hugs closely the west- 

 ern side of the plain, and flows close under the base of the Coast 

 Ranges, the building up of low confluent alluvial fans, by the rivers 

 embouching on the* plain from the Sierra Nevada, having forced 

 the main San Joaquin River further and further west. The latter 



* Whitney Geology of California, pp. 55 and 188. 



