14 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
Coast System. The course of this mountain front participates in the curvature, with 
convexity to the Pacific, observable in the land portion of the Coast Ranges, in the Great 
Valley of California, and in the Sierra Nevada. This convexity toward the Pacific is, 
it may be observed in passing, characteristic of the dominant tectonic lines about the 
border of that great ocean. It is very marked in the Aleutian belt, in Kuriles, in the 
Japanese Isles, in the festoon extending from Formosa thru the Philippines, the Moluccas, 
and Java to Sumatra, which is convex to both the Pacific and the Indian Oceans; and 
in the chain including the Salomon Islands, the New Hebrides, and New Zealand. It 
is also apparent in the trend of the Sierra Madre Occidental and Sierra Madre del Sur of 
Mexico, and in the course of the Andes thru Colombia, Ecuador, and Peru. 
Having indicated the east and west boundaries of the Coast System as their dominant 
structural lines, we may now consider those features which pertain to the internal struc- 
ture of the mountain tract. Here we must first take note of the coast line. The coastal 
slope of California characteristically rises abruptly from sea level to elevations of from 
2,000 to 5,000 feet within a short distance from shore, from Cape Mendocino to Point 
Conception, with certain notable breaks in its continuity which are susceptible of special 
explanation. If along the shore line at the base of this abrupt slope we draw straight 
lines which are tangent to the headlands or chords to the minor embayments of the 
coast, these lines fall into two fairly constant orientations and clearly bring out the 
fact that the shore line has in reality a zigzag course, due apparently to the alternate 
control of two systems of structural lines, one of which is between N. 37° W. and N. 40° 
W., and the other between N. 10° W. and N. 15° W., thus intersecting at an angle of 
about 26°. Under this scheme of discrimination of the orientation of different portions 
of the coast line, the bearings of the following divisions may be thus listed: 



; > BEARING OF DISTANCE IN GEO- 
ee Bera Bea MEAN LINE. GRAPHICAL MILES. 
Cape Mendocino to Punta Gorda . N. 12°. W. 14 
Punta Gorda to Shelter Cove . N. 40° W. 25 
Shelter Cove to Point Arena ieee N. 10° W. 64 
Point Arena to Golden Gate, thru Tomales 
Bay TORN Ee ee te een Poe N. 40° W. 90 
Golden Gate to Pigeon Point ; N. 15° W. 40 
Pigeon Point toward Santa Cruz . N. 40° W. 21 
Point Pinos to Point Sur N. 138° W. 19 
Point Sur to Port Hartford . : N. 37° W. 89 
Port Hartford to Point Conception Nee 6: We 44. 


Now it is difficult to regard any considerable portion of the abrupt coastal slope of 
California between Cape Mendocino and Point Conception as other than a more or less 
degraded fault-scarp. If this view be accepted, it is clear that the trend of the coast and 
its geomorphic profile have been determined by two systems of faults meeting or inter- 
secting at an angle of about 26° on their strike. Making some allowance for cliff reces- 
sion, the base of both systems of scarps must lie some little distance off shore and be 
buried by the notable embankment of littoral sediments which conceals the true profile 
of the submarine rock surface. 
Of the two systems of faults thus recognized as controlling the trend of the coast, one, 
viz. that which bears N. 37° W. to N. 40° W., conforms, as will be shown later, more or 
less closely with the prevailing structural lines, such as faults, folds, and belts of igneous 
rock found in the Coast Ranges; while the more meridional system is not a prominent 
feature of the Coast Ranges. It follows that since the mean trend of the California coast 
lies between the bearings of the two fault systems, the tectonic lines of the Coast System, 
if followed northwesterly, eventually emerge upon the coast. This obliquity of the 
