THE SAN ANDREAS RIFT AS A GEOMORPHIC FEATURE. 47 
valleys would carry it into the San Bernardino Range at the point where Mission Creek 
emerges upon the wash plain. Continuing still farther northwest, we follow a marked 
topographic break which leads across the southern slope of San Gorgonio Peak to the 
head of Mill Creek. It is very probable that the great fault followed so far joins the 
above fault-line at some point easterly from Palm Spring Station, altho the many miles 
of gravel-covered desert makes a positive statement impossible with present knowledge. 
An examination of the northerly and easterly base of San Jacinto shows conditions 
opposite to those characterizing the southern slope of the San Bernardino Range. Ero- 
sion is generally slow upon the slopes of San Jacinto, while the rapid erosion from the 
opposite side of San Gorgonio Pass has crowded the stream channels close to the base of 
the former range. In fact, the base of the San Jacinto Range appears to be deeply buried 
by the stream deposits. The desert face of San Jacinto has long been free from disturb- 
ances. Long, jagged ridges project out into the desert, while the intervening canyons, 
instead of furnishing material for extensive débris fans, are floored by accumulations 
characteristic of the desert as a whole. 
Toward the southern end of that spur of the San Jacinto Mountains which projects into 
the Colorado Desert and is known as the Santa Rosa Mountains, the débris fans are larger 
and remains of gravel deposits appear high up on the sides of the mountains. The only 
suggestion that a fault traverses the Salton Basin in the direction of the mouth of the 
Colorado is the presence of mud volcanoes and several small pumiceous eruptions near 
the center of the basin. These are, however, so far removed from any known fractures 
in the crust that their evidence is of little value. Besides, it is entirely possible that the 
mud voleanoes may be due to chemical action in the deeply buried sediments of the Colo- 
rado delta. 
It may be reasonably assumed, then, from our best knowledge, that the southern end 
of the great Rift is to be traced for an unknown distance along the base of the mountains 
bordering the Salton Basin upon the northeast, in all probability gradually dying out. 
SAN JACINTO FAULT. 
The San Jacinto fault (plate 30), with which there has been associated at least one 
severe earthquake since the region has been known, has a length of at least 75 miles. 
The course of the fault is northwest and southeast, and it is marked by canyons or steep 
mountain scarps nearly its whole length. The fault first appears upon the south in the 
form of a regular mountain wall inclosing the north end of Borego Valley. The latter 
is a western arm of the Colorado Desert lying between the Santa Rosa Mountains and the 
main watershed of the Peninsular Range. At the northern end of Borego Valley beds 
of late Tertiary age appear faulted down upon the southwest side of the mountain wall 
referred to. The peculiar topographic features of this fault-block ridge, and the presence 
of gravels along portions of its summit, make it appear of recent origin. Northwest of 
Borego Valley the canyons entering Coyote Creek have brought down immense quantities 
of rock débris, a fact which indicates recent disturbance along their headwaters. Ter- 
williger Valley includes a broad expanse of country of low relief upon the summit of the 
range between San Jacinto and Borego Valley. A portion of the valley is scarcely drained 
at present, having apparently undergone some subsidence next to the fault-line which 
forms the southern face of Mount Thomas. 
In a northwesterly direction, the fault can be traced in continuous mountain scarp 
or canyon until within about 8 miles of the town of San Jacinto. A broad valley inter- 
venes until we get north of the town, when a mountain wall commences again, and extends 
for many miles in the direction of Colton. Reports state that the San Jacinto earthquake 
of 1899 was most severe along the line of the fault thus traced. Great masses of rock are 
