THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. 55 
category with those found farther south, merely on the strength of their superficial 
resemblance. 
The effects of a horizontal shear on thick grass sod in open country, as observed in a 
number of localities along the zone of faulting in Sonoma and Mendocino Counties, are 
as follows: On fairly level ground, where conditions are simplest and no vertical move- 
ment is evident, the sod is torn and broken into irregular flakes, twisted out of place and 
often thrust up against or over each other. The surface is thus disturbed over a narrow 
belt, whose width apparently varies with the magnitude of the displacement. Along 
the main fault, where the throw amounts to 10 feet or more, a width of 5 or 6 feet is not 
uncommon; on the secondary fractures, where the throw does not exceed a foot, the belt 
is generally only a foot wide. Whatever the width of the belt, the sod within it, as well 
as the unconsolidated material underneath, appears loosened up and not compact. It 
consequently takes up more space than before it was disturbed, and the surface of the 
belt is therefore slightly raised above the level of the ground, from an inch to a foot or 
more, according to the magnitude of the disturbance. Within such a belt there is sel- 
dom, if ever, a well-defined, continuous, longitudinal crack, the toughness of the sod 
precluding a clean shear fracture. Rather, there is a marked predominance of diagonal 
fractures resulting from tensile stresses. 
To sum up, a horizontal displacement produces and may therefore be recognized in 
grassy country by a fault-trace showing: 
1. The disturbance of the sod over a narrow belt. 
2. The generally raised surface of this belt. 
3. The absence of a single continuous, longitudinal crack. 
4. The tearing of the sod along numerous diagonal fractures. 
5. The twisting and thrusting of sod flakes against and over each other, 
It was mainly by the aid of these criteria that the fault lines in the vicinity of Shelter 
Cove were determined as fault or shear fractures, distinct from the innumerable cracks 
due to the settling of earth masses consequent upon the jar of the disturbance. In 
practically all of these the sod had been ruptured by mere tension, or tension accompanied 
by more or less vertical shearing. Furthermore, as will presently appear, the location 
of the fault fractures was in many instances facilitated by their association with the 
characteristic fault topography observed all along the line. 
What appears to be the main fault-trace was first observed in the bottom of Wood 
Gulch, where it runs immediately along the wagon road for a hundred feet or more. It 
was thence traced south to its southern terminus on the beach of Shelter Cove, and 
then north across Humboldt Creek up Telegraph Hill. Subsequently several apparently 
detached lines of a similar character were discovered in the neighborhood of the main 
fault, as shown on the sketch map. Beginning at the south end, this line may be traced 
as follows: 
On the beach of Shelter Cove, 100 yards west of the frame hut of Snider (at the mouth 
of Deadman Gulch), the fault passes thru the bluffs obscured by dislocated masses of 
dark conglomerates. From the top of the escarpment, however, it is easily traced for 
some distance down. The approximate contour map of the fault (fig. 10) sufficiently 
describes the topography here. A notable feature is a small elongated pond on the 
steep hillside, walled in by a small ridge. Thru this the fault-trace passes longitudinally. 
Continuing north, the line remains east of the upper valley of Wood Gulch until it joins 
the wagon road in the bottom. The line is by no means straight, as the bearings on the 
map indicate. A pronounced angle in its course exists at A (fig. 10), and the coincidence 
in this change of azimuth with the abrupt topographic change at this point is strongly 
suggestive of a hade to the west. Near the loop in the road at B the fault is easily recog- 
nized, except where the road has been repaired. The fault-trace here passes thru a 
\ 
