Page:Popular Science Monthly Volume 69.djvu/75

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71
THE SAN FRANCISCO EARTHQUAKE

PSM V69 D075 Section of the seven mile beach and beyond showing the faults.png

Fig. 2. Section of Seven-mile Beach and Beyond; from mouth of Lake Merced, showing position and exposure of Merced series of rocks. This remnant of this series lies between the San Bruno fault plane at the north and the San Andreas fault at the south. Its entire absence from the San Bruno Mountains to the north and Mount Montana to the south is part of the basis for the theory that since its deposition there has been uplift along the two faults which lifted the terrritory outside of these faults nearly or quite a mile and one half above sea-level, and that erosion, not only planed down the folded rocks of this block, but entirely removed the Merced rocks either side of this block.

 

PSM V69 D075 Recent marine deposits above seven mile beach.png

Fig. 3. Figure showing Position of Recent Marine Deposits above Seven-mile Beach and the relative movement along the two fault planes.

 

Since then two events are clearly shown in the records; these are subsidence and differential uplifts. The subsidence carried most, if not all, of the San Francisco peninsula below its present elevation, flooding the valleys and leaving the hills of the city largely an archipelago. Marine deposits were laid down on top of the clearly recognizable sand dunes and wash deposits, containing in many places the trees mentioned above. Following that came local uplift, raising these marine deposits to elevations of over 700 feet above sea-level just

 

PSM V69 D075 Mussel rock beach showing pleistocene sand and rocks.png

Fig. 4. Mussel Rock from the South, showing marine Pleistocene (a) overlying sand dunes (c) and igneous rocks (b).