260 
The San Andreas fault is a break in the earth’s crust that extends 
for many miles across southern and central California. Movement 
along it began far back in the Tertiary period and has progressed at 
intervals to very recent time.®! It passes along the southwest side 
of the Mecca and Indio Hills and traverses the valley-fill deposits in 
the intervals between these ridges, where in places it gives rise to a 
low cliff. This feature is well shown in the airplane photograph re- 
produced in Plate 39. Its course, as recently determined by L. F. 
Noble, is shown on sheets 27 and 28, together with that of another 
similar break known as the Mission Creek fault, which joins it near 
Indio. 
The fault trace is less conspicuous along the south side of the Mecca 
Hills, where in places it is marked by a low bluff, extending as far as 
Mortmar siding. It is believed by Noble to continue southeastward 
under the Salton Sea to the mud volcanoes southwest of Niland and 
thence southeastward by Brawley and Holtville. Another fault 
beginning in the Indio Hills is believed to extend through Dos Palmas 
and Frink Springs and continue approximately parallel to the railroad 
northeast of the sand-hill belt. There are scarps and springs in 
places along its course. 
North of Indio the fault extends along the southwest margin of the 
Indio Hills nearly parallel to the railroad and from 2 to 3 miles distant. 
The older crystalline rocks of the high mountains bordering the Colo- 
rado Desert and Coachella Valley are schists and gneisses penetrated 
by old granite. These schists and granites are cut by younger 
granitic igneous masses and overlain by a younger series of schists, 
limestones, and quartzites that are considerably metamorphosed. 
(Brown, Vaughan, and Frazer.) 
GUIDEBOOK OF THE WESTERN UNITED STATES 
the east side of which basal conglomer- 
ates rise on the mass of schist that 
appears at Shaver Well. 
The Indio Hills have practically the 
same structure as the Mecca Hills, 
except that they consist caddalhe of two 
anticlines in a faulted block cut off on 
the southwest by the San Andreas 
fault. (Noble, L. F., personal com- 
tion. 
munica 
8t That there still is movement along 
this fault or other faults west of it 
Bull., vol. 5, pp. 130-148, 1916.) In 
order to e the amount of 
vertical movement on this line of 
displacement precise levels have been 
through El Centro, Niland, Yuma, and 
Jacumba, a distance of 158 miles. 
These, when compared with previous 
levels, indicate slight vertical displace- 
ment a short distance south of Niland 
(probably on an extension of San 
Andreas fault), just south of Brawley, 
and farther south on the supposed east- 
ward continuation of the Elsinore fault. 
earthquake of March, 1932, which 
caused Ih 1 oie Dp ane Coa 5 eee 
near Long Beach, was due to movement 
that centered in the ocean, to the west. 
