THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. 69 
plate 44.) A few minor branches were seen on the east side. The pool or lane of 
water shown in plate 42s is about 2 feet deep. Mr. Shafter states that the ground here 
was dry and under cultivation before the earthquake. Shortly after the shock he noticed 
that the current of a creek close by was reversed. 
Just south of the head of Tomales Bay, Papermill Creek enters the valley from the east, 
crosses to the southwest side of the valley and then turns toward the bay, in which 
it has built a delta. The delta occupies the whole width of the bay and is about 3 miles 
long, the greater part of it being submerged at high tide. At the head of the delta Olema 
Creek joins Papermill, bringing its tribute of detritus; and on the opposite side of the 
valley Papermill Creek receives the water of Bear Valley Creek, which brings no sediment 
but filters for some distance through a marsh. At the head of the delta a road crosses 
the valley, resting partly on the delta and partly on the marsh just mentioned, and fur- 
nished with an embankment to lift it above the floods. Just before reaching this road 
the fault-trace enters the marsh, where it quickly expands to a width of nearly 60 feet 
and exhibits the trench phase. Not only was the road offset by the fault and earthquake, 
but the portion between the walls of the trench was dropt down, the embankment sink- 
ing into the soft earth until nearly flush with the marsh. In restoring the embankment 
about 3.5 feet of earth were added. Close to the road Papermill Creek was crost, with 
offsetting of banks, and thence the fault was continued thru the delta to its end. (See 
plate 464.) Its course is nearly straight and of such 
direction as to pass just outside the end of the cape 
near Millerton, the bearing being N. 35° W. At several 
points it is margined on the northeast by a lane of 
water (plate 45a), indicating that a narrow tract 
on that side is deprest, but no evidence was found of 
a general depression of land on one side of the fault as 
in Bolinas Lagoon. The echelon phase is dominant; 
the ridge phase does not appear. The trench phase 
obtains for short distances, and is combined for larger 
distances with the echelon. Where the trench phase 
occurs, it coincides with the zone of abundant cracks and is thus distinguished from 
the sag holding the lane of water. 
The general relation of the sag to the fault-trace is shown in fig. 17. It occurs only on 
the northeast side, but is so persistent that, from a commanding position, the fault can be 
traced out by means of the water lanes. The depression will probably average more than 
50 feet wide, but it eludes measurement because it fades out gradually on the side away 
from the fault. The greatest noted depth is 17 inches, but the average is probably less 
than a foot. In attempting to interpret this feature I assume that beneath the smooth 
plane of the delta, and buried by its soft deposits, is a variegated topography of the rift 
type; and the hypothesis I advance is that the new-made sag on the delta plain is the 
surface echo of a fault-sag of the buried topography which was made deeper by the event 
of April, 1906. It has already been pointed out (page 67) that the sags of the Rift 
which were touched by the new fault were apparently deepened; and if the true explana- 
tion of the delta-sag has been suggested, we have in that feature an indication that the 
deepening was not only apparent but real. 
At the northern edge of the Inverness settlement is an outlying or branch fault-trace 
about half a mile long. (Plates 458 and 47a.) Starting in what is called the “Second Val- 
ley,” it ascends to a mesa and then descends toward the “Third Valley,” its course being 
about N. 20° W. In crossing the upland it is associated with a fault-sag and there exhibits 
the trench phase with horizontal displacement of 2.5 feet. ‘Two shorter traces, trending 
northward, occur on the slope between the “First Mesa” and the “Second Valley.” 










Fic. 17.—Hypothetic section of fault 
under Papermill Delta, 
