Ransome.] 



The Great Valley, 



387 



the heavy local accumulation of marine Pliocene in the Coast 

 Ranges, it is likely that by far the greater part of the latter was 

 above or very close to sea-level. During the Pliocene, with the 

 Coast Ranges thus depressed, and the present site of the Sierra 

 Nevada occupied by a peneplain of much gentler slope than now 

 the bottom of the Great Valley stood at a relatively greater elevation 

 than at present, with reference to its inclosing rim, and may have 

 been partly above the sea-level, and certainly could not have been far 

 below it. We know, too, that during the latter part of the Pliocene, 

 the rivers flowing down over the Sierra Nevada peneplain, were 

 actively engaged in depositing gravels along their lower courses. 



Finally, at the close of the Pliocene, occurred the oft-described 

 elevation of the Sierra Nevada by a tilting of the old peneplain 

 towards the west (2. e., towards the Great Valley), and closely follow- 

 ing it the gradual emergence of the Coast Ranges, which has con- 

 tinued through the Pleistocene to the present time, as ably shown 

 by Professor Lawson. The obvious effect of this double uplift, along 

 two nearly parallel axes, was to deepen the Great Valley, and give 

 it substantially its present form. To quote from Professor Lawson, 

 " Numerous islands, large and small, fringed the coast of California. 

 There were numerous submerged valleys, so that the coast was well 

 supplied with harbors. In a word, the coast of California at the 

 close of the Pliocene, had the aspect of an archipelago. The archi- 

 pelagic condition endured into the early Pleistocene, and from this 

 condition it has been gradually recovering up to the present day."* 

 An exception to this recovery was noted by the same writer in a 

 later paper, however, in which it is shown that a local subsidence to 

 the extent of 378 feet, and "entirely subsequent to the general uplift 

 of the coast," has affected the region of San Francisco Bay.f 



It will be seen from the foregoing, that up to the beginning of 

 the Pleistocene, the area now occupied by the Great Valley was at 

 no time the theater of particularly heavy sedimentation or active 

 subsidence. There is evidence that during the Cretaceous and 

 Miocene the heaviest sediments and deepest depressions lay to the 



* Loc. cif., pp. 158, 159. 

 t Ibid, p. 265. 



