MINOR GEOLOGICAL EFFECTS OF THE EARTHQUAKE. 395 
the tendency of the vibrations being to set the mass in motion. Earth-flows occurred 
in many places in the Coast Ranges, and probably thruout the region in which the 
shock was heavily felt. The writer found many of them, large and small, on the San 
Francisco Peninsula and in the Santa Cruz Mountains, also in the Mount Diablo and 
Mount Hamilton Ranges. 
Following are descriptions of 5 earth-flows that occurred on the morning of the earth- 
quake in the neighborhood of Half Moon Bay, which is on the coast 25 miles south of 
San Francisco : 
One of them was formed in the hills bordering the terrace at Half Moon Bay, immedi- 
ately south of Frenchman Creek, 1.5 miles north of the town, and a mile from the sea, 
at an elevation of 100 feet. It is pictured in plate 1324. At this place the earth caved 
away in a crescent-shaped area on a slope of only 18°, and flowed out in two long arms 
so as to leave a hole 4 feet deep, surrounded by vertical walls of unaffected soil. The 
flow occurred at a fairly high point ona gently undulating incline. The discharged earth 
was divided by a mound, at a point 150 feet below the summit of the arc, and followed 
two courses which were determined by gullies on both sides. Much of the débris over- 
flowed the central mound at the same time, and inundated the barley fields to a depth 
of 2 to 4 feet, for 100 feet farther. On both sides of the central mound the caving away 
continued to the same depth. In the left-hand fork it stopt within a few feet, and the 
flow did not extend very far beyond. In the right-hand fork a cut 100 feet long and 
50 feet wide was made, the earth flowing down from it 250 feet farther over the grain 
field, as shown in plate 1324. Thus the whole length of the slide was 500 feet. The 
width of the main hole was on the average about 100 feet, and the length, as already 
mentioned, 150 feet not including the arms. 
In this hollow in the hillsides many dry blocks of sod carrying growing grain — usually 
in an upright position — were left stranded 4 feet below the surface of the hill by the 
removal of the subsoil. The fence that crost this area was broken and carried away 
and partly buried. Where the caving ceased in the right fork, a ridge of débris was 
piled up across the mouth of the hole, much higher than the stream of loose material 
that flowed farther. Similar ridges were heapt up across the path of the flow, where 
the breaking away of the hill stopt in the other arm and at the upper end of the central 
mound. 
The south or right arm of the flow extended down the hill at an angle gradually 
decreasing from 18° to less than 5°. Large parts of the fence were carried on its sur- 
face for 300 feet. 
Plate 132 gives a detailed view of the lower extremity of the right arm. The stream 
came to an abrupt stop, like a quickly cooled lava flow, and preserved a face 1 to 2 feet 
in height above the grain field. The surface of the flow consisted largely of blocks of 
sod, usually almost upright, which were carried down from the hole without much 
moistening, or transformation into material capable of flowing. The bulk of the flow 
was a moist aggregate of earth fragments possessing something of their previous form 
and grading into mud, which assumed a semi-fluid consistency underneath. The 
bottom of the hole, and the flow itself, remained too muddy to walk on for wecks after 
the earthquake, and the field below the lower end of the large arm was left marshy, 
tho it had not been so before. It is to be noted that several fairly heavy rains followed 
the earthquake after an interval of several days, and before these earth-flows were 
visited; but these were not sufficient to account for the amount of moisture observed. 
The chief effect of the water was in the ground at a depth of 3 or 4 feet below the sur- 
face. It rendered the soil sufficiently fluid to enable it to flow down the gentle slope, 
probably partly oozing from under the surface crust and partly transporting the sod 
with it. Most of the surface was carried down with the main flow, the stranded sur- 
