Page:Popular Science Monthly Volume 69.djvu/78

74 other evidence that there has been sudden slipping along these fault lines in very recent years, comparable in importance with movements of two months ago, especially as expressed in fault scarps? According to the preliminary report of the earthquake committee, the rupture of April 18 shows a horizontal displacement averaging 10 feet, and a vertical displacement not to exceed four feet. How long such a surface disturbance can be subsequently recognized is a question. The San Andreas fault belt is well situated for the noting of any such displacement for several miles southeast of Mussel rock, in many cases the actual fault planes emerging in sublevel pasture land at the surface. Neither the writer's notes nor his memory now yield any evidence of such a scarp. On the other hand, the fact that where such evidence might be seen has long been subject to the tramping of cattle renders its absence of less value. In this connection he does not give any value to a small scarp noted just back of Mussel Rock. At the time, it was considered to be a land slip. A photograph taken of it suggests the possibility of its having a deeper meaning.

But if such scarps are lacking, there is abundant evidence of another kind bearing on this subject. I have spoken of the San Andreas fault belt. Such it appears to be rather than a single clearly-defined break. Along this belt between San Andreas lake and where the belt meets the ocean at Mussel Rock is a string of drainless depressions occupied with water part or all of the year. In one or two cases these can be clearly seen to lie directly in one of the lines of faulting. That they are the result of fault movements seems highly probable. When were they made? That they were made within the last few centuries can not be asserted, yet the fact that so many of these shallow basins still exist, neither filled nor drained, notwithstanding that in many cases it is but a stone's throw to the head of a drain with a high gradient, suggests such a possibility. The possible cause of these basins is suggested in what appears to have formerly been one, now trenched from two directions at the head of Wood's Gulch, a small ravine cutting the cliffs of Seven Mile beach, a mile north of Mussel Rock. The ravine follows a fault with downthrow of 800 feet. At the head a cirque-like cut exposed an overhanging fault scarp of 100 feet or more. Against this face there appears to have gradually filled in wash from the adjacent hills, wind-blown sands and detached fragments from the fault face, until the whole thing was buried and later covered with the marine deposits of the last submergence. Judged from what is left of this filling, it must at one time have strongly resembled the undrained basins just described. The evidence suggests that this fault scarp was produced by a single movement. An elephas tusk found about

75 feet from the top of the filled in deposit agrees with the other evidence in placing the time of this movement back to the land period preceding the recent submergence.