THE SAN ANDREAS RIFT AS A GEOMORPHIC FEATURE, 49 
the mass of the mountain blocks. Farther south, near Bitterwater, the Rift leaves the 
northeast flank of the Gavilan, and lies along the southwest base of a straight ridge of the 
Mount Hamilton Range. Still farther south in Cholame Valley it follows the northeast 
base of the ridge which separates Cholame Valley from San Juan Valley. In the Carissa 
Plain it hugs the southwest flank of the Temblor Range. But the most noticeable rever- 
sal of the relative position of the Rift to the adjacent mountain slopes is beyond Tejon 
Pass. From Tejon Pass to near Cajon Pass, the Rift lies along the steep northerly flank 
of the San Rafael and San Gabriel Ranges, on the southern edge of Mojave Desert; but 
at Cajon Pass it passes thru between the San Gabriel and San Bernardino Ranges, and 
thence easterly lies on the south side of the latter range. Thus from the San Francisco 
Peninsula to its southern end, so far as the extent of the Rift is at present known, there is 
a fairly regular and rather remarkable alternation of the relative positions of the Rift 
and the mountains adjacent to it. 
The Rift as a whole, when plotted upon a general map of the state on a scale of about 
Tooeo0D BpPpears as a sensibly even line with marked curvature, convex toward the 
Pacific. This curvature is for the most part due to change in the course of the Rift be- 
tween the southern end of Carissa Plain and Tejon Pass. In this segment of its course its 
trend changes from about 8. 40° E., along the edge of Carissa Plain to 8. 65° to 70° E., 
along the southern edge of the Mojave Desert, the change being gradual and distributed 
over an are about 40 miles in length. The general curvature is also accentuated by the 
change in course between Point Arena and Shelter Cove, on the assumption of continuity 
between these points. If, however, we take the segment of the Rift between Point Arena 
and the south end of Carissa Plain, the curvature is very much less marked; and its path 
on the small scale map referred to approximates a straight line. The curvature is dis- 
tinct, however, and, as in the general case, is convex to the Pacific. The chord of the are 
found by stretching a line from the south end of Carissa Plain to the mouth of Alder 
Creek has a bearing of about N. 40° W. and a length of about 360 miles. The point on 
the arc most distant from this chord is on a normal to the latter thru San Jose, the 
distance being about 15 miles. 
When the Rift is plotted on larger scale maps (see maps Nos. 2, 4, 5, 21, and 23), it 
becomes apparent that the course of the Rift is not a smooth uniform curve, but is charac- 
terized by several minor curvatures in opposing directions. In locating these curves, 
advantage is taken of the fault-trace, as far as it extends, as a sharp line within the Rift 
indicating its mean trend at any point on its course. These curvatures are most interest- 
ing features on a line of diastrophic movement, where that movement may be, as it was 
on April 18, 1906, essentially horizontal on a nearly vertical plane or zone. 
The most northerly curvature susceptible of measurement is that shown by the seg- 
ment of the Rift between the mouth of Alder Creek and Fort Ross. The line connecting 
the two ends of this segment, at the points where it intersects the shore line, is a little 
more than 43 miles in length, and has a bearing N. 37° W. The Rift, as located for this 
purpose by the fault-trace, lies wholly on the southwest side of this chord. The bearing 
of the fault-trace at the mouth of Alder Creek, where it converges upon the chord, is 
N. 30° W., and at Fort Ross its bearing is N. 40° W. The fault-trace is at its maximum 
distance from the chord about the middle of this segment, the distance being about 0.75 
mile, and here the bearing of the Rift is sensibly the same as that of the chord, N. 37° W. 
Between Fort Ross and Bodega Head, where the Rift passes under the Pacific, there is 
probably a slight reversal of this curvature; since, if the course of the fault-trace at Fort 
Ross were continued, even as a straight line, it would pass to the eastward of the point 
where it actually intersects the neck of the headland. ‘This slight concavity to the south- 
west probably extends as far as the mouth of Tomales Bay. From Bodega Head south 
thru Tomales Bay to Bolinas Bay, the course of the Rift as a large geomorphic feature is 
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