THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. 59 
POINT ARENA TO FORT ROSS. 
For the course of the fault and the phenomena of earth movement along it for the 
stretch of 43 miles between the mouth of Alder Creek and the point on the shore south 
of Fort Ross where it passes beneath the Pacific, we are again indebted to Mr. F. E. 
Matthes, who, on behalf of the Commission, made an examination of this territory. In 
the vicinity of Fort Ross, however, several observers contributed notes as to the phe- 
nomena there. For this entire distance, the rupture of the ground and its differential 
displacement are strongly marked and, except for the occasional local obscuration of the 
phenomena by brush and timber, are easily traced. The fault-trace enters the shore 
less than half a mile north of the mouth of Alder Creek and crosses with a course of 
S. 28° E. the bench-land, or wave-cut terrace, to the banks of the creek about 500 
feet in from its mouth (fig. 12). Over the surface of the bench it is marked by char- 
acteristic rending and heaving of the sod. At the point where it reaches Alder Creek, 
the stream bank is rocky and steep, and the course of the crack can be traced down the 
rocky bluff, tho somewhat obscured by talus. The face of the bluff is shown in plate 32a. 
On the edge of the bench above the stream cliff (B, fig. 12), there is a rocky knob project- 
ing above the general level. The earth crack passes close to the southwest side of this 
knob. The hade of the crack on the face of the bluff for a height of about 50 feet is very 
nearly vertical, but its deviation, if any, from the vertical could not be accurately meas- 
ured on account of the ragged character of the bluff and the loose rock upon its face. On 
the northeast side of the rocky knob above referred to, there is evidence of a less well- 
marked parallel crack, as indicated on the sketch (fig. 12). This also appears on the 
rocky bluff of the stream cliff, but is less distinct than the main crack. 
Southeast of this point, the fault-trace fol- 
lows the broad stream bed of Alder Creek for 
nearly a mile, passing beneath a bridge, the 
wreck of which is shown in plate 328. In this 
view, the horizontal offset of the bridge along 
the fault-line is well shown. It is apparent 
that this offset is not less than the width of the 
bridge. On the southwest side of the stream, 
near the bridge (A, fig. 12), the fault-trace is 
flanked by peculiar, isolated, rocky knobs simi- 
lar to that on the northeast side. It is not 
clear, however, that these rocky knobs have - 
more than an accidental relation to the fault, 
since they may possibly be residual sea-stacks 
upon the uplifted wave-cut terrace. 
After leaving Alder Creek, the phenomena of 
surface rupture and displacement were traced 
thru a series of ranches to the divide passing 
over to Brush Creek, and down to the vicinity = ee 
of Manchester. Along nearly this entire dis- be Seas ae Lane: Airset 
tance between Alder Creek and Brush Creek, 
the line passes thru a series of depressions, swamps, and ponds, the majority of 
which are not connected with the neighboring streams. Offsets due to the displace- 
ment were measured on two fences of Mr. E. E. Fitch’s ranch, and the amount of move- 
ment was found in each case to be 16 feet, the southwest side having moved relatively 
toward the northwest. The vertical displacement was, as a rule, quite small; only in 
a few places did it amount to a foot, presenting a low scarp of that height facing the 
northeast. To the north of Manchester, an east-west fence line was offset in three places, 




‘Scale (approx) 
1000 feet 
a 

Cc I 


