Ransome.] 



The Great Valley. 



389 



face of the ground only 46 feet above sea-level, the occurrence of 

 logs of wood at 340 feet, recent shells up to 600 feet, and coarse 

 gravels at depths of over 2,000 feet, can only be accounted for upon 

 the supposition of an equal subsidence. Numerous beds of coarse 

 gravel could have been deposited in the middle of this broad valley 

 only when its bottom was close to, or above, sea-level. If, as sup- 

 posed by the geologists of the U. S. Geological Survey, the Great 

 Valley was occupied by a shallow lake during early Pleistocene 

 times, then the fluviatile deposits under Stockton would seem to be 

 of even later date. But probably then, as now, the valley was mainly 

 above sea-level. The streams flowing into it may have formed local 

 and transient lakes through the damming up of the drainage by 

 delta-formations, as is seen in the case of Tulare Lake to-day. 

 During the wet seasons, extensive floods probably spread the finer 

 materials widely over the floor of the valley, as is done on a smaller 

 scale by the existing rivers. There is the possibility, too, that the 

 elevation of the Coast Ranges may have been at times more rapid 

 than the corrasion of the outlet of the valley at Carquinez Straits, 

 thus leading to a temporary, but more or less general, flooding of 

 the Great Valley with fresh water. 



Finally, the subsidence in the middle of the Great Valley, as 

 shown by borings, is not to be considered alone, but evidently 

 stands in some connection with the more extensive local sag in 

 which the Bay of San Francisco lies, and which has been described 

 by Antisell* and Lawson.| To sum up briefly, we have during 

 Pleistocene times a progressive general elevation of the rim of the 

 basin in which the Great Valley lies, first by the tilting of the Sierra 

 Nevada crustal block, and second by the epeirogenic uplift of the 

 Coast Ranges as a whole. At the same time, sediments are being 

 deposited in the valley aggregating over 2,000 feet in thickness in 

 its middle portion, with an accompanying pari passu subsidence or 

 depression of the valley floor. Coeval with this subsidence we have 

 a marked sag across the Coast Ranges in the same latitude, result- 

 ing in the flooding of San Francisco Bay. 



The Great Valley is an example of a well-defined area of pro- 



* Loc. tit., p. 27. 

 t Loc. tit., p. 265. 



