GEOLOGY OF THE COAST SYSTEM OF MOUNTAINS. 13 
straight, but is curved, with the concavity toward the northeast. Between the southern 
end of the valley and the vicinity of Coalinga its course is about N. 35° W. From 
Coalinga, where there is an offset or jog in the general trend north to Tracy, the course 
is about N. 30° W. From Tracy to Suisun there is a marked westerly embayment in 
the Coast Ranges which is probably due, in part at least, to the depression of the region 
about the Bay of San Francisco. From Suisun northward to the vicinity of Red Bluff 
the general course of the margin of the Coast Ranges is north and south. At Tejon 
Pass the eastern margin of the Coast System receives the abutment of the southern 
end of the Sierra Nevada; thence southward, with a course swinging more easterly, it 
determines the southwest limit of the Mojave Desert. 
On the seaward side the Coast System is usually regarded as being limited by the shore 
line. The precipitous coast rising to elevations of from 2,000 to 5,000 feet, extending 
from Cape Mendocino to Point Conception, and the popular notion that mountain ranges 
are confined to the land areas of the earth, are justification for this view. But in a more 
comprehensive view, embracing all inequalities of the earth’s surface both above and 
below the sea-level, the western margin of the mountainous area, the familiar portions 
of which we call the Coast System, will have to be placed farther seaward. Off the coast 
of California the sea-bottom slopes down to the 3,000-foot submarine contour at a mod- 
erate angle and then plunges steeply to depths of over 12,000 feet. Beyond the foot 
of this steep slope the sea-bottom has very flat gradients and the 15,000-foot contour is 
far out to sea. From the Oregon line to Point Conception the 3,000-foot submarine con- 
tour, or the brink of the steep slope, lies off shore at a distance of from 15 to 35 miles; 
but at Cape Mendocino and at the Bay of Monterey this line is found much closer in. 
South of Point Conception this steep slope has the same general trend as to the north. 
That is to say, it shows no embayment in its course corresponding to that at the Santa 
Barbara channel and southward. ‘This is particularly true of the course of the 6,000, 
9,000, and 12,000-foot contours. The slope is by no means uniform for its entire length. 
From Point Arena to the latitude of the Golden Gate the grade is notably steep from 
the 3,000 foot to the 9,000-foot contour. This is also true off Point Conception. From 
the latter point southeastward the steep portion of the slope is from the 6,000-foot 
to the 12,000-foot contour; and the same statement holds for the slope off San Simeon 
Bay. In general, the steepest profile lies between the 6,000 and the 9,000-foot line. 
This steep drop from the subcontinental platform to the broad floor of the Pacific 
must be regarded as the geomorphic expression of a rather acute deformation of the 
earth’s crust, and those portions of the slope where the contours are crowded together, 
as for example between Point Arena and the latitude of the Golden Gate, off San Simeon 
Bay, off Point Conception, and off the platform of the Channel Islands, can scarcely be 
interpreted as other than fault-scarps. The slopes at the localities mentioned are quite 
comparable to the great fault-scarp which forms the eastern front of the Sierra Nevada. 
At the base of the slope off the Channel Island platform, the recent dredging operations 
of the Albatross brought up from a depth of 12,000 feet numerous fragments of rock 
similar to the bituminous shale of the Monterey series of the southern Coast Range. 
With this rock was found much asphaltum. This indicates that at the base of the slope 
there are talus accumulations of so recent a date that they have not yet been buried by 
oceanic sediments. 
This line of acute deformation of the crust off the entire length of the coast of Cali- 
fornia can not be ignored in any consideration of the orographic features of the region. 
The slope referred to is doubtless devoid of those sculptural features characteristic of 
mountains within the zone of erosion, and which we are too apt to look upon as essential, 
but it constitutes nevertheless a notable mountain front rising from the floor of the 
Pacific. It is the natural western boundary of the mountainous tract which we call the 
