THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. ‘We 
east of it, but mostly in the district to the west, where all of the country roads were more 
or less obstructed. 
On the west side of the main ridge west of the head of Tomales Bay there occurred 
two wet slides. In one case a hillside bog was loosened from the slope on which it rested 
and descended as a flow of mud to a canyon bottom 100 or 200 feet below. In the other 
case the earth beneath a wet meadow in a rather steep canyon flowed down the canyon 
for about 0.5 mile, overpowering trees on its way and leaving a deposit 15 or 20 feet 
deep in places. This was the largest individual slide observed. 
In all the cases mentioned the conditions were such that slides would have taken 
place at some time had the earthquake not occurred. But this statement may not 
properly apply to the cases about to be mentioned. 
On the steep southern face of Mount Tamalpais a number of rocks were loosened and 
rolled down the slope, some of them being large enough to cut swaths thru the thicket 
which were visible for months afterward. Similar swaths were seen, under a crag in the 
vicinity of Willow Camp. In the bottom-lands of creeks it happened at many places 
that a slice of the alluvium was separated by a crack parallel to the bank and slid into 
or toward the stream. In some cases alluvium lying with a gentle slope adjacent to a 
marsh slid toward the marsh, opening a crack along its upper edge. 
Mention has already been made of numerous hillside cracks which marked incipient 
landslides. In such cases the downward motion apparently began during the earth- 
quake agitation, but the momentum acquired was not sufficient to continue the motion 
after the earthquake stopt. In a very large number of these localities motion was 
resumed and landslides occurred during a period of excessive rainfall in the spring of 
1907. (Plates 544 and 55a.) So far as my observation goes, all of the landslides having 
this history were wet, the material usually flowing freely down the slope as a thin mud. 
The probable explanation is that the cracks made in April, 1906, served to admit the 
water flowing over the surface during the rains of 1907, so that the material which was 
too dry to flow in 1906 acquired the proper consistency and continued its course the fol- 
lowing year. The number of landslides which the earthquake induced in this indirect way 
is possibly as large as the number which were an immediate consequence of the shock. 
The phenomena of landslides bring to attention certain conditions of flow which affect 
a variety of earthquake features. Consolidated formations hold steep slopes by virtue 
of cohesion. Incoherent formations maintain the “slope of repose”? — 380° to 35° — 
by virtue of the resistance to sliding, or the static friction, of their particles. Certain 
formations, including some clays and clay mixtures, become coherent by drying and 
incoherent by wetting. Incoherent formations, as a rule, have a less coefficient of 
friction when wet than when dry. For these reasons the addition of water is the ordinary 
immediate cause of a landslide; it overcomes cohesion, or else it reduces the frictional 
resistance, and slipping or flowing is the result. During a strong earthquake, agitation 
overcomes the cohesion of feebly-coherent formations and suspends the operation of 
static friction between the particles of incoherent formations, thus affecting the materials 
somewhat as water affects them. In the case of landslides, it may enable an incoherent 
dry formation to flow as if wet; and it may temporarily give to an incoherent formation, 
wet or dry, a condition of quasi-liquidity. 
Ridging and shifting of tide lands. —'The general width of Tomales Bay near its head 
is about a mile, tho it is constricted at one point by a promontory jutting out from the 
east shore. (Fig. 24.) Papermill Creek, entering at its head, has built a delta which 
slopes so gently toward the deeper water that the tides range over it for a distance of 
about 3 miles. The upper half of the slope is covered by vegetation of various kinds 
and the lower half is of bare mud: In the region of vegetation the soil has sand as well 
as mud, and the bed of the stream is of sand and gravel. As the delta deposit has been 
