Page:Popular Science Monthly Volume 69.djvu/76

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72
THE POPULAR SCIENCE MONTHLY.

PSM V69 D076 Raised beach near lobetus.png

Fig. 5. Raised Beach near Lubetus, Quaternary overlying Merced Strata.

 

about San Francisco, and double that farther down the coast. These last movements were differential, and it is quite possible that in some cases the two took place together. In the same way, it is quite possible that while in parts of the area the evidence suggests elevation continuing at present, elsewhere subsidence is in progress, though not so evident. The point of special interest in this connection is the fact that this differential movement about San Francisco is clearly a movement of the fault blocks and reveals slipping along these planes of faulting of hundreds of feet in times so recent as to suggest that much of it may have taken place since the human occupation.

Considering first the evidence of movement, a good example is furnished by the San Andreas fault, where it reaches the ocean at Mussel Rock. At the foot of the parallel San Bruno fault a few miles to the northeast the recent marine deposits are about at sea-level. Coming south along the sea-cliff or southwest, these deposits are clearly seen rising, making distinct benches in the little stream valleys where they overlie the wind and wash deposits of the preceding stage, these wash deposits in places being full of half exposed spruce logs, which the neighboring inhabitants use for firewood. Before the San Andreas fault is reached, these deposits have risen to over 700 feet above sea-level, lying on the truncated upturned edges of the Merced strata. As soon as the trace of the San Andreas fault has been crossed, the top of these sediments is found at only 220 feet above sea-level, whence it descends until within a short distance it reaches the level of the beach. Here is therefore clear evidence of movement along the fault plane of several hundred feet since this last submergence. The question of most interest then becomes: How recent was this last uplift?