ORIGIN OF SMALL FAULT TROUGHS ALONG ACTIVE FAULTS 667 



rivers.^ This might account for circular and more or less irregular- 

 shaped depressions, but hardly for rectilinear trenches with such sharp 

 steep sides and flat bottoms that they suggest canal excavations. The 

 hypothesis is certainly inapplicable to the depressions mentioned in the 

 present paper. 



Gilbert, in explaining troughs at the base of the Wasatch scarp, as- 

 sumed that the fault-plane separating the alluvium from the firm rock 

 of the mountain had a dip of 60 degrees, and that the recent uplift of 

 the mountain block relative to the alluvium resulted in a break which, 

 as it approached the surface, curved upward away from the fault-plane 

 so as to leave a triangular prism of alluvium attached to the rock consti- 

 tuting the footwall. The open fissure formed by faulting along the up- 

 curving surface would be immediately filled by the settling of one or 

 both walls.^ 



The objections to this hypothesis are: (1) that depressions are absent 

 for long distances where there is a single fault-scarp instead of several 

 branching fractures and there is no other evidence of open fissures having 

 been formed; (2) that pressure effects, such as striations, are found 

 along the fault surface in gravel cemented by calcite as well as on the 

 solid rock; (3) that at the only place w^here the fault-plane was observed 

 in solid rock the dip was as steep or steeper than in the alluvium and 

 glacial drift; and (4) that it is not applicable to similar troughs along 

 the San Andreas fault where the displacement was horizontal, or to 

 those formed in solid rock during the earthquake at Yakutat Bay, 

 Alaska.io 



Later, Gilbert explained the sags and ridges along the San Andreas 

 fault in the Bolinas-Tomales Valley as due to "the unequal settling of 

 small crust blocks along a magnified shear zone," ^^ but he advanced no 

 theory to explain the settling. The horizontal displacement of 1906 was 

 accompanied in places by slickensides and similar evidence indicative of 

 compression normal to the fault rather than of tension. 



Oldham attributed the fomiation of certain depressions during the 

 Assam earthquake of 1897 to the lurching of alluvium at the base of hill 

 slopes, but such depressions follow the windings of the contact between 

 hill and plain.^- 



8 M. L. FuUer : The New Madrid earthquake. U. S. Geol. Survey, Bull. 494, 1912. 

 pp. 48-58. 



» G. K. Gilbert : Lake Bonneville. U. S. Geol. Survej-, Monograph I, 1890, pp. 355-356. 



1° R. S. Tarr and L. Martin : The earthquakes at Yakutat Bay, Alaska, in September, 

 1899. U. S. Geol. Survey, Prof. Paper 69, 1912, p. 37, and plate XVII, C. 



11 G. K. Gilbert : "Characteristics of the rift" in the California earthquake of April 

 18, 1906, vol. i, pt. 1, 1808, pp. 33-34. 



^ R. D. Oldham : Report on the great earthquake of 12th June. 1897. Mem. Geol. 

 Survey India, vol. 29, 1899, pp. 92-93. 



