Proceedings of the California Academy of Sciences, Volume III, 1863-1867

by Various

Collected by Mr. Herbert C. Dorr in Nevada Territory.

Dr. James Blake read a paper on the gradual elevation of the land in the environs of San Francisco.

On the Gradual Elevation of the Land in the Environs of San Francisco.

BY JAMES BLAKE, M.D., F.R.C.S.

The gradual elevation and depression of large portions of the earth’s surface has, within the last few years, been attracting considerable attention from geologists. It is a vast geological process of which we are the actual spectators, offering us the most imposing terrestrial phenomenon of which we can be cognizant, and at the same time affording us some tangible idea of the vast periods that have been required for bringing the surface of the earth to its present shape. It is the general opinion of geologists that the western shore of our continent is gradually rising. This has been proved to be the case as regards the southern portion of the continent; but the following facts, observed in the neighborhood of this city, afford undoubted evidence that at least this portion of the northern continent is being gradually elevated above the level of the ocean.

On the northern bank of Lobos Creek, a small stream running from Mountain Lake to the ocean, muscle shells and rolled pebbles are found at an elevation of from eighty to one hundred feet above the present level of the ocean, and probably at the distance of half a mile from the present beach. These shells and pebbles are exactly analogous to those now being deposited at the mouth of the creek, and were undoubtedly placed there when the spots at which they are found formed the beach of the ocean. The surface of the country is so much covered by drifting sands, that it is only in spots that these shore remains show themselves. The deposits first seen contain remains of shells considerably weathered—lower down the creek, shells and larger pebbles are seen; still lower down I found the same materials mixed with smaller pebbles, and at an elevation of about fifty feet small bands of black peat earth were found interstratified with the sand and gravel. These small bands of vegetable earth were evidently formed near the level of the ocean by the waves throwing up a barrier of sand which dammed up the waters of the creek, so as to form a pond in which a layer of vegetable matter was deposited. This process is going on at the present time, a dam having been thrown up by the heavy storms of the winter of 1861-1862.

Another evidence of the recent elevation of the country is seen near the western end of the Puerta Suelo, at a distance of about eight miles from the city. Here there is a depression in the hills, extending from the bay to the ocean, and forming a narrow neck to the peninsula on which San Francisco stands. Even at present, the distance from the waters of the bay to the ocean is not more than two or three miles at this point, and it is evident that at no very distant period this depression formed a channel of communication between them. Near the western end of this former channel, and at about a mile inland from the present sea beach, the skeleton of the head of a whale is found on the surface of the ground. The specimen measures about six feet across, and must have belonged to an animal fifty or sixty feet long. The bones, which are not at all mineralised, are in a good state of preservation. At the time they were carried there, there must have been eight or ten feet of water over the surface, and as the place is at present from ten to fifteen feet above the level of the ocean, a rise of twenty-five or thirty feet must have taken place at this spot since the animal was washed there.

Another locality at which evidence of the gradual elevation of the land can be obtained is found to the west of Black Point, where abundant remains of our present bay shells are found at a considerable elevation above the level of the sea; and, were not the surface of the country, particularly the lower levels, so completely covered in by the drifting sands, no doubt many analogous deposits could be found. To the south and west of the Mission, and in all the lower levels between there and the range of hills overlooking the Puerta Suelo, the surface is covered by these recent post tertiary deposits, through which the older rocks protrude in many places as isolated masses, the recent argillaceous sandstone being deposited in nearly horizontal strata around their base. These sandstones have given rise, by their decomposition, to the extensive surfaces of yellow sandy loam seen between the Mission and the Ocean House. I think the highest of these beds does not attain a greater elevation than one hundred feet above the present level of the ocean.

More recent evidence of the gradual elevation of the land is furnished by the holes made by the marine worms in the rocks on the shores of the bay, many of these holes being found at elevations which the highest tides do not at present reach.