Page:Popular Science Monthly Volume 69.djvu/77

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73
THE SAN FRANCISCO EARTHQUAKE

Farther south on the ocean side of the San Francisco peninsula the coast is fringed with a raised beach, such as occur so abundantly along much of the California coast. The top of this beach is 75 to 100 feet above sea-level. At Purisima, the creek of that name runs out across this raised beach through a slightly cut channel to the very edge of the plane where it drops as a water-fall directly into the surf. As the rocks through which it would have to cut to reach sea-level are the only partly consolidated Merced series, there is striking evidence of the recency of the raising of the beach. Again just south of San Francisco are many places in which the recently raised deposits and in slighter degree the underlying Merced rocks have been trenched to depths of as much as 75 feet since the production of the Coast and Geodetic Survey map of this region in 1869. The fact that so large a part of these recent deposits still remains in view of this rapid erosion impresses one in the field most profoundly.

There is another line of evidence to which the writer refers with some hesitation. Indian shell mounds abound along the California coast. In many cases these now occur spread out in thin sheets, apparently forming the surface layer of the raised beaches over such large areas that in his first study of them he was deceived and considered them in many cases as marine deposits. As he remembers them now, he can not help thinking that in many cases they have been reworked by water before the final uplift. In one case 'the writer found an Indian skeleton, evidently formally buried, half exposed in the side of a stream channel so narrow as to force one to the conclusion that the channel has been entirely cut since the burial of the body. This is only a fraction of the evidence that in the field leads one to consider this last uplift as a thing of yesterday, and in all probability of to-day also, or, in other words, that these differential uplifts are still in progress.

A final question of maximum interest is: Is there physiographic or

 

PSM V69 D077 Usgs map from mussel rock towards san andreas lake.png

Fig. 6. A Portion of the U. S. Geological Survey Map from Mussel Rock southeastward nearly to San Andreas Lake, showing topographic environment of a chain of ponds and undrained basins believed to be due to recent earthquake movements.