THE SAN ANDREAS RIFT AS A GEOMORPHIC FEATURE, 
GENERAL. 
Extending thru the greater part of the Coast System of mountains from Humboldt 
County to the Colorado Desert, a distance of over 600 miles, is a line or narrow zone 
characterized by peculiar geomorphic features, referable either directly to the modern 
deformation of the surface of the ground or to erosion controlled by the lines upon which 
such deformation has taken place. This peculiar feature has been known, both to Cali- 
fornian geologists and to residents of the sections where its characters are most promi- 
nent, but its extent and importance were not fully appreciated until after the earth- 
quake of April 18, 1906. It is commonly reported among the residents of the southern 
interior Coast Ranges, particularly in San Benito, Monterey, and San Luis Obispo 
Counties, that displacement of the ground occurred on this line in the earthquake of 
1857 and in certain later earthquakes. The first reference in scientific literature to this 
feature appears to have been in the year 1893, in a paper entitled “‘The Post-Pliocene 
Diastrophism of the Coast of Southern California,” by Andrew C. Lawson, which is 
quoted in the sequel. The next reference to this peculiar line is in the eighteenth annual 
report of the U. 8. Geological Survey for 1896-1897, Part IV, in a paper by Schuyler 
on “ Reservoirs for Irrigation,’ where, pp. 711-718, the significance of the line is fully 
recognized in the following words quoted in full: 
This reservoir has especial interest, not only as the first one of any magnitude com- 
pleted on the Mojave Desert or Antelope Valley side of the Sierra Madre in southern Cali- 
fornia, but because it lies directly in the line of what is known as “the great earthquake 
crack” of this region, which is marked by a series of similar basins behind a distinct ridge 
that appears to have been the result of the great seismic disturbance. 
This remarkable line of fracture can be traced for nearly 200 miles thru San Bernardino, 
Los Angeles, Kern, and San Luis Obispo Counties, and deviates but slightly here and there 
from a direct course of about N. 60° to 65° W. There appears to have been a distinct 
“fault”’ along the line, the portion lying south of the line having sunken and that to the 
north of it being raised in a well-defined ridge. In many places along the great crack, 
ponds and springs make their appearance, and water can be had in wells at little depth 
anywhere on the south side of the ridge before mentioned. A tough, plastic, blue clay 
distinguishes the line of the break, in this portion of its course at least; and where the 
line crosses Little Rock Creek, the blue clay has formed a submerged dam, which has 
forced the underflow near the surface and created a “ cienega”’ immediately above it. After 
crossing the line, the water of the creek drops quickly away into the deep gravel and sand 
of the wash. The same effect is noticeable at other streams, and it has been suggested as 
the probable cause of the very distinct rim marking the lower margin of the San Bernardino 
Valley artesian basin and confining its waters within well-defined limits, as this rim is nearly 
on a prolongation of the line that is traceable on the north side of the mountains — the 
break having crost the mountains thru the Cajon Pass on the line of Swartout Canyon. 
In 1899 the essential features of the same line in the region north of the Golden Gate 
were recognized and discust by F. M. Anderson. In later years Dr. H. W. Fairbanks 
has traced out the line in various field trips and has given several public lectures 
descriptive of its features and its significance, but has published no systematic account 
of his studies. 
The fact that the earthquake of April 18, 1906, was caused by a rupture and dis- 
placement of the earth’s crust along this line for a distance of about 190 miles, immedi- 
ately focussed the attention of local geologists upon it. Among those engaged upon 


1 The Geology of the Point Reyes Peninsula, Bull. Dept. Geol., Univ. Cal., vol. 2, No. 5, p. 143 et seq. 
Anderson, however, supposed, as is indicated by the last paragraph of his paper, that the faulting ante- 
dates entirely the Pleistocene terrace formations. 
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