THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. 65 
uted over a zone 415 feet in width. Another fence farther southeast on Doda’s ranch, 
having a bearing of N. 36° E., was offset on the fault-line 15 feet; the southwest side, as 
usual, having moved relatively to the northwest. This fence is shown in plate 348. One 
of the most interesting effects of the displacement due to the fault is that seen where 
the latter intersects a small stream at Doda’s ranch-house. The stream flows trans- 
versely to the line of the fault, and has a trench across the terrace about 5 feet deep. 
On the lower or southwest side of the fault, the stream trench has been moved north- 
westerly about 12 feet, so as to bring a fault-scarp across the entire width of the upper 
part of the trench and impound its waters in the form of a pool. The result is shown in 
plate 37a and also on Mr. Matthes’ map of the Rift at this place (map No. 3). The 
impounding of the waters on the line of the fault is interesting evidence of the absence 
of any open crack. 
BODEGA HEAD TO TOMALES BAY. 
The location of the fault across the neck of land which connects Bodega Head with the 
mainland was determined by Prof. J. N. LeConte. He reports that on the south side 
of this neck the main earthquake fissure was found passing about 50 yards west of a house 
occupied by Mr. Johnson. It could be traced as a multitude of small cracks in the swampy 
land from the bay to the road, then as a well-defined fissure up the small depression west 
of the house for 200 yards to where it disappeared in the sand dunes. No trace of it 
could be detected in the sand dunes, which reach from this point entirely across the penin- 
sula. Only one fence crosses the fissure and this had been repaired so that no measure- 
ment of the displacement was possible. The movement was evidently northward on 
the west side, as was shown by the direction in which the bushes were bent. The vertical 
movement was about 18 inches, the uplift being on the west side. The sand spit which 
closes the bay on the south was examined for evidence of movement, but nothing could 
be detected in the drifting sand. 
At the mouth of Tomales Bay there are two points projecting westward from the east 
shore, and both of these, according to the observations of Prof .R. 8. Holway, are crost 
by the fault-trace. The first is a long, flat sand-spit extending well across the mouth of 
the Bay just south of Dillon’s. The line of the fault was still visible in the sand on 
June 11, 1906, in spite of the obliterating action of the wind and the recent rains. The 
line lies near the base of the spit and has a northwest-southeast course. On each side 
of the crack are crater-like depressions, some of them being double or overlapping. 
Mr. Keegan, the owner of Dillon’s Beach, reported that these craterlets were numerous 
and distinct. In some instances a great deal of sand and water had been ejected. Others 
are reported on the southwest side of the fault-trace, from which the belt containing them 
extends some 70 feet. The craterlets vary in size up to 6 feet in diameter and it is 
reported that on the day after the earthquake the water which stood in them could not be 
bottomed by a fishing pole. | 
About 1.5 miles southeast of this spit is a promontory about 100 feet high projecting 
into the bay. Some 400 yards from the end of this promontory on top of the ridge is a 
liné of depression with two or three small ponds. The main fault fissure here divides into 
two cracks, one each side of this depression, which is about 150 feet in width. Stand- 
ing on this ridge, the line of the fault can be traced at low tide for nearly 1.5 miles across 
the bottom of the bay to the sand-spit to the northwest, its course in general being parallel 
to the axis of Tomales Bay. (See plate 38c.) The horizontal displacement where the 
fault crosses the promontory is about 8 feet, as determined by the line of tall grass at the 
edge of the little ponds, the westerly side having shifted to the northwest. 
F 
