MINOR GEOLOGICAL EFFECTS OF THE EARTHQUAKE. 397 
The largest of the earth-flows seen occurred in the canyon south of the house of Mr. 
Nunez, 2.5 miles east-northeast of the town of Half Moon Bay, at an elevation of about 
500 feet. It originated in a manner similar to the others, but in a canyon along which 
there is a distinet but ordinarily dry stream channel. <A long, irregular hole from 4 to 
7 feet deep was excavated near the head of the valley, and a great volume of earth 
flowed down its curving course for 0.25 mile, as far as the Nunez house, and there stopt, 
being in part diverted into the main ereek to which the valley is there tributary. Ac- 
cording to the testimony of witnesses, the flow reached the end of the 0.25 mile in 0.5 
hour after the earthquake shock. It was seen gliding slowly down and engulfing the 
orchard just back of the house. According to observers on the Nunez ranch, the earth- 
flow was not accompanied by any water; but two wecks later, when examined by the 
writer, it preserved every evidence of having been muddy. lispecially was this true 
at the bottom, where great masses of mud stili had the consistency of jelly. It is prob- 
able that there was no flowing water on the surface of this or other earth-flows at the 
time of their formation, and that the presence of water in the flow was not evident to the 
casual observer because of the comparative dryness of the material on its upper surface. 
The slope of the canyon down which the moving body of land crawled is about 25° 
near the head and decreases to 15° farther down. The flow filled this to a width of 
100 feet on the average, and to a depth varying from 10 to 20 feet. The inertia of the 
mass is illustrated by the fact that in the early stage of the flow the earth was piled 
20 feet higher on the hill, on the inside of the big curve made by the canyon, not far 
below the pit, than it was when the flow came to rest. The marks at this elevation 
were probably made very soon after the main mass was discharged from the cavity, 
before it had spread very widely. The central portion of this earth-flow is pictured 
in plate 1318, where it appears as a ridge many feet high rising above the tall erass on 
the hillside, on the right of the picture. The pressure of the material at the head of 
the flow, as it started, was so great that the earth bulged up over the sides in places, 
in such a way as to force upward great blocks of sod and turn them on edge or com- 
pletely over, away from the rim of the hole. 
The flow assumed the form of two lateral ridges and a central depression, or channel. 
The ridge on the west or inner side of the curve was considerably the higher. The 
form was due partly to the concavity of the valley; but chiefly, it is thought, to the 
tendency of the more fluid material to follow the deepest: possible path along the gully 
under the center of the flow. Thus the drier material was retarded at the sides. Sub- 
sequent to the first starting of the flow, a stream of semi-fluid mud and sand continued 
to run down the central channel, covering its sides with a coating of mud and leaving 
flowage striations on it. This channel and its markings are exhibited in plate 131s. 
Two weeks after the earthquake, when the photograph was taken, water was running 
in this channel and had cut down into it several feet deeper. Its bottom, however, was 
still from 5 to 10 feet higher than the bottom of the underlying preéxistent water 
course, where water had not flowed before at this time of the year. The man in the 
picture is standing at the bottom of the gully. To the left of him, the hammer and 
note-book mark the top of one of the parts of the lateral ridge which is here divided 
into several hummocks. To the right is the other and higher lateral ridge. The fore- 
ground was formerly covered by a dense thicket of willow trees. These willows have 
been completely buried, except at the sides where some dead branches protrude. A 
fence that crost the canyon was torn away for 100 feet, and not a trace of it could be 
found. The fence shown in the picture is one newly built in its place. 
Two other smaller earth-flows occurred just over the hill westward from the last one 
described. They are shown in plate 1338, the canyon on the left being the one occupied 
