Page:Popular Science Monthly Volume 69.djvu/110

From Wikisource
Jump to: navigation, search
This page has been proofread, but needs to be validated.
106
THE POPULAR SCIENCE MONTHLY.

fillings, some of the cracks opened by passing waves do not close again, but remain as permanent vestiges of the shock. Closely associated with these secondary cracks in soft ground are permanent changes in surface form. At the head of Tomales Bay, for example, a broad tract of soft ground between high and low tide was thrown into low ridges, with cracks along their crests, and these remained until destroyed by wind waves. In San Francisco considerable tracts of 'filled' land were shaken together and thus made to settle a few feet, and were at the same time slidden several feet toward the bay (Fig. 9).

Certain changes, very conspicuous to the observer who drove about the country, are closely associated with roads. A side-hill road is

PSM V69 D110 Earthquake fracture crosses a road near bolinas.png

Fig. 7. A Zone of Earthquake Fracture where it crosses a Road near Bolinas.

 

usually constructed by excavating a notch in the natural slope and piling the excavated material in an embankment at the outer edge of the notch. In course of time, and especially during rainy seasons, the embankment at the outer edge of such a road settles and has to be built up as a matter of repair. Portions of the bluff on the up-hill side of the notch are also apt to fall away, taking the form of small landslides, which have to be removed from the road as a rule after every rainy season. The earthquake precipitated many changes of this sort. Along all side-hill roads in the immediate vicinity of the rift a crack was developed between the embankment and the original soil against which