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University of California. 



[Vol. i. 



west of the present valley, and along the line now occupied by the 

 Coast Ranges. During the post-Miocene upheaval of the latter, 

 the folding, as is generally the case, was confined to the zone of 

 exceptionally thick sediments, while the floor of the Great Valley 

 seems to have remained as a passive, inert mass between the folding 

 Coast Ranges and the Sierra Nevada. Such subsidence as it may 

 have experienced during these periods would be but a marginal 

 facies, as it were, and directly dependent upon the far greater sub- 

 sidence which allowed the heavy accumulations to take place which 

 were subsequently folded up into the Coast Ranges. It is therefore 

 a corollary to the problem of the formation of these ranges, and of 

 mountain building in general, which, although it may be regarded 

 as an isostatic problem, is not the particular one with which we are 

 immediately concerned. 



But with the post-Pliocene elevation of the crest of the Sierra, 

 and with the gradual upward diastrophic movement of the Coast 

 Ranges during Pleistocene times, whereby the Great Valley was 

 finally cut off from the sea, we are brought directly to the problem 

 in hand. The valley became closed in by mountains as we find it 

 at the present day, and became a definite and well-bounded area of 

 sedimentation or deposit. The movements which brought about 

 this condition are, as far as we know, still going on. The great 

 orographic block of the Sierra Nevada is still being tilted up at a 

 higher angle, and the Coast Ranges (with perhaps one or more local 

 exceptions) are still emerging from the sea. All through Pleistocene 

 and recent times, the streams flowing down from the Sierra, and from 

 the eastern slope of the Coast Ranges, have been pouring detritus 

 into the deepening valley, depositing the coarser materials in broad 

 alluvial fans, and carrying the finer silt farther out, to be spread over 

 the plain in flood seasons. We have seen that at Stockton, in the 

 middle of the valley, borings to a depth of from 2,000 to 3,000 feet 

 remain entirely within unconsolidated fluviatile deposits. The age 

 of all these clays, sands, and gravels is not definitely known, but it 

 seems most probable that they are all post-Miocene, or even post- 

 Pliocene. There is nothing, unless it be their great thickness, which 

 points to any but a very recent geological age for the deposits, as 

 far as they have been penetrated by borings. With the present sur- 



