MINOR GEOLOGICAL EFFECTS OF THE EARTHQUAKE. 387 
EARTH-AVALANCHES. 
Warth-avalanches were caused chiefly along the sea-cliffs of the coast on the morning 
of the earthquake, tho some also occurred on steep canyons within the zone of high inten- 
sity. On the coast the earth-avalanches were for the most part simply an exceptional 
incident in the normal process of cliff recession. Where the upland of the Coast Ranges 
approaches the shore, the horizontal corrasion of the waves maintains a steep sea- “cliff; 
and the recession of the sea-cliff is effected by the repeated occurrence of earth- avalanches 
due to the undermining by the sea, combined with the disintegrating action of atmos- 
pheric agencies. Pers are thus always upon the face of the cliff masses of earth or 
rock, the fall of which is imminent and may easily be precipitated by a severe shock of 
earthquake. 
The most notable of the earth-avalanches occurred where the sea-cliffs are highest and 
steepest. This happens on the coast of Humboldt County, between Cape Mendocino 
and Point Delgada. Not only are the cliffs here particularly favorable for large earth- 
avalanches, but the coast here is close to the line of the fault which caused the earth- 
quake, and so received an exceptionally severe shaking. For many miles of coast there 
was a general slipping of rock and earth into the sea, down very precipitous sea-cliffs 
ranging up to over 2,000 feet in height. Between Shelter Cove and Point Arena, the 
sea-cliffs are not so high nor so continuous, but there was nevertheless a very general, 
and locally large, shedding of material from their face; and the sea was muddy for many 
days after the earthquake in consequence of the dejection of the débris upon the shore, 
within range of the attack of the waves. 
From Point Arena southward to Fort Ross, the cliffs are low, being for the most part 
not in excess of 100 feet. Harth-avalanches were nevertheless of common occurrence 
along this stretch of coast. South of Fort Ross to Bodega Head the cliffs are again, 
as far as the mouth of the Russian River, several hundred feet high and very steep. 
Here again earth-avalanches were extensive. The rocks along this entire stretch of 
coast from Cape Mendocino to Bodega Head are prevailingly sandstones and shales. 
On the sea-cliffs on the north side of Bolinas Bay and west of the town of Bolinas, there 
was a very general crumbling and fall of the sea-cliff upon the beach. South of the 
Golden Gate, the most notable earth-avalanches were along the sea-cliffs between the 
eity and Mussel Rock. This cliff has a length of about 6 miles and ranges in height 
from about 100 feet up to 700 feet, and is cut almost wholly in the strata Bf the Merced 
(Pliocene) series, which are inclined at angles varying from 15° to 75°. The rocks are 
for the most part rather soft and (esherent, tho there are numerous well-cemented 
and indurated beds in the series. This cliff converges on the fault at a small angle, and 
intersects it at its south end near Mussel Rock. The cliff was severely shaken and great 
quantities of earth and rock were caused to fall or slip down. The great. earth-slump 
at Mussel Rock (Plate 129c, D) was also notably accelerated. A similar sudden 
movement of the ground occurred on the west side of Merced Lake, whereby a large sec- 
tion of the slope sank toward and into the lake, and a portion of the bottom of the lake 
was lifted above the surface by the deformational rotation of the collapsed ground. 
To the south of Mussel Rock there were several small earth-avalanches along the 
cliffs, and numerous cracks were formed near the brink of the cliffs which will in future 
doubtless lead to further falls from the cliff-face. Near San Pedro Point there was a 
large movement of the earth on the face of the high cliff. One earth-avalanche to the 
north of the Devil’s Slide started about 800 feet above the shore and swept the face of 
the cliff, carrying away several hundred feet of roadbed. The slide occurred near the 
contact of sandstones reposing on granite, and both kinds of rock were involved. Smaller 
earth-avalanches occurred farther south on the sea-cliffs. 
