THE SAN ANDREAS RIFT AS A GEOMORPHIC FEATURE. 45 
Asylum, there is a much dissected fault cliff 200 to 300 feet in height. Plainly traceable 
in the front of this cliff is a small break, possibly made in 1857. No definite information 
could be gained as to whether the earth opened here at that time, but reports say the 
earthquake was very severe, throwing animals from their feet, ete. 
East of City Creek begins a huge rounded ridge formed in the mesa gravels, and this 
ean be traced nearly to Plunge Creek. This ridge is 150 feet wide and steeper upon its 
upper side, where the greatest displacement shown is about 40 feet. The structure and 
shape of the gravel ridge make it appear likely that faulting and folding were both con- 
cerned in its making. Above this ridge and at the highest point where it crosses the mesa, 
water is obtained in abundance for irrigation at a depth of 18 to 20 feet, while in the 
mesa below the ridge no water is found at a depth of 200 feet. 
The Santa Ana River has cut out a wide stretch of the mesa gravels, and has exposed 
upon its eastern bank a good section of these gravels. The gravels at their upper edge 
do not lap over the crystalline rocks but appear faulted down against them. A 0.25 mile 
below the fault is the mouth of Morton Canyon, the stream issuing thru a long, narrow 
canyon eroded in the mesa gravels. Morton Canyon emerges from the steep mountains 
about 2 miles to the southeast and has taken this peculiar course thru the gravels to the 
Santa Ana River, instead of flowing directly down across them, as do all the other streams. 
The explanation of the turning to the northwest of this canyon at the point where it 
meets the gravels is found in the peculiar appearance of the gravel slope when viewed in 
profile. This, instead of rising with normal slope, becomes steeper toward the upper edge, 
and then descends abruptly to Morton Canyon. The movement on the Rift has broken 
and lifted up the gravels to such an extent that the waters of Morton Canyon were 
diverted and turned down to the Santa Ana River along the upper side of the ridge. Since 
this displacement took place, they have had time to cut the canyon in which they are 
now flowing. Southeast of the point where the Rift crosses Mill Creek, the peculiar topo- 
graphic features which have characterized it for so many miles become very indistinct. 
It was at first thought that the Rift terminated in this vicinity but closer examination 
made it clear that such is not the case. 
The southern portion of the San Bernardino Range lying between Mill Creek and the 
Conchilla Desert appears to have undergone great disturbance at a recent date. As a 
consequence, erosion has been rapid and extensive, and surface features which farther 
north made the Rift easy to follow have in this region been almost completely obliterated. 
Potato Canyon extends along the line of the Rift to the southeast of Mill Creek. Its 
features indicate that the history of the fault is a complex one. The canyon originated 
thru erosion upon the fault contact between the crystalline rocks of the San Bernardino 
Range and the older Pleistocene deposits along its base. Following this period of ero- 
sion was one in which gravels were again deposited and this was succeeded by the present 
period in which erosion is active. Potato Canyon is the last of the longitudinal depres- 
sions of any size marking the line of the Rift. Between its head and the desert to the 
southeast the main drainage features pay little attention to the structural conditions, 
because of the steep grades of the stream channels and consequent rapid erosion. Never- 
theless small lateral canyons have been formed along the fault contact of the gravels 
with the crystalline rocks of the higher portion of the San Bernardino Range, so that from 
the proper viewpoint the fault line can generally be traced in the topography. The 
drainage of Potato Canyon is clearly influenced by the fault, for instead of there being one 
stream course in it, there are two — one upon each side. 
A mile southeast of Oak Glen, which is at the head of Potato Canyon, there are large 
springs which issue upon the line of the fault. Near this point a depression appears upon 
a gravel ridge, where it meets the crystalline rocks. The depression is in line with the 
course of the fault, and may with reason be attributed to dislocations similar to those so 
