THE SAN ANDREAS RIFT AS A GEOMORPHIC FEATURE. 33 
sinking a few inches on the side toward the middle of the valley. Thus the surface 
changes associated with the earthquake tended, within this belt, to increase the dif- 
ferentiation of the land into ridges and valleys; and it is easy to understand that the 
inception as well as the perpetuation of the ridges and valleys was due to faulting. 
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Fic. 5. — Cross-profiles of fault-sags. 
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Fic. 4.— Map of side-hill Fic. 6.—Cross-protile of side-hill 
ridge. sag shown in plates 7B and 10B. 
Collectively these ridges and valleys occupy a belt from 0.5 to 1 mile in width, and 
constitute the local development of the Rift, using that term in its narrower sense. They 
make up the entire surface of the belt, except where overpowered by some vigorous 
creek. The individual ridges are not of great length, being 2 or 3 miles at the most, 
and usually much less. Some of them end by wedging out, others by dropping down 
until replaced in the same line of trend by valleys. Their greatest height above base, 
except where the adjacent valleys have been deepened by erosion, is about 150 feet. 
The narrower have straight, acute crests; the broader have undulating backs with 
more diversity of form than is shown by the associated valleys. Some are crost by 
curved or straight depressions, and these depressions have all the characters of the 
parallel valleys, including the association of earthquake cracks. 

Fie. 7.—Cross-profile of Bolinas-Tomales Valley. Vertical and horizontal scales 
the same. RR=limits of Rift. P=valley of Pine Gulch Creek running SE. 
O= valley of Olema Creek running NW. 
In the remainder of this report the term Rift will be applied only to the narrow belt 
just described. Regarding it as the surface expression of a great shear zone or compound 
fault, the ridges are the tops of minor earth-blocks, and the valleys are in part the tops 
of relatively deprest blocks and in part depressions resulting from the weathering of 
crusht rock. Considering the Rift as a physiographic type, I find it convenient to have 
a specific name for one of its elements, the small valley; and in some of the descriptions 
which follow I shall speak of it as a fault-sag. (See plates 7B, 8a, and 11.) 
The general relation of the Rift to the greater valley is illustrated by the cross-profile 
in fig. 7. Along its northeastern side it everywhere lies lower 
than the adjacent slope of the greater valley, the produced 
profile of the valley slope passing above the fault-ridges as well 
as the fault-sags. Along its southwestern side some of the fault- 
ridges appear to project above the restored profile of the greater 
valley, while the fault-sags lie below. If I interpret the struc- Fre. 8.—Ideal section 
ture correctly, the great compound fault concerned in the making faite cae ae 
of the valley trough —a fault of which the vertical dislocation 
amounts to several thousand feet—includes a certain amount of step-faulting, which 
D 
