MAGNETOMETER SURVEYS IN CALIFORNIA 1357 
the surface because of the loose soil covering the surface. An erratic dip 
of 40° N. in the south flank lends support to this small fault. 
All the vectors of disturbance, when plotted at each station, point 
toward the axis of the fold, the angle of each vector with the horizontal 
becoming steeper as the axis is reached until, at the axis, the vectors are 
almost perpendicular. 
WHITE CREEK SYNCLINE 
This syncline, 20 miles northwest of Coalinga, trends N. 60° W. for 
a considerable distance, but the locality chosen for the magnetometer 
line was in the SE. 14, Sec. 23, T. 19 S., R. 13 E. (Fig. 3). 
The Cretaceous beds are folded into an asymmetrical syncline with 
the north flank dipping 65° and the south 45°. Folded in the trough 
of these older beds and overlapping them is the Etchegoin. The 
axis of the syncline in the Etchegoin can be determined on the ground 
because of the outcropping on both sides of the axis of the characteris- 
tic Glycimeris bed. The Etchegoin is moderately magnetic and the 
Cretaceous is strongly magnetic. 
Results—The plotting of the vertical intensities and the magnetic 
vectors of disturbance showed a condition the reverse of those plotted 
from the Raven Pass anticline. The verticai intensities, plotted in gam- 
mas, showed a curve with its lowest point at the axis of the syncline where 
the Etchegoin would be at its thickest over the magnetic Cretaceous. 
The highest points in the curve naturally occurred at the end of the 
line directly above the Cretaceous. 
The vectors of disturbance point vertically down at the axis; and 
outward from this point toward the more magnetic Cretaceous beds. 
WALNUT CREEK FAULT (FIG. 4) 
The Walnut Creek area comprises an alluvial valley bounded on the 
west by Martinez Ridge, and on the south by Shell Ridge, both of 
which expose Tertiary rocks. The structural features consist of two 
major faults traversing the valley, approximately at right angles to each 
other, and several minor faults. On the northeast side Cretaceous rocks 
are exposed and the valley is surrounded on the other two sides by the 
Eocene rocks. This is an ideal condition, as the Cretaceous in Walnut 
Creek is very magnetic, the Eocene is weakly magnetic, and these for- 
mations are cut by several faults. 
The work was begun by running lines with the magnetometer across 
the supposed location of a major fault, marked on the map as the Mar- 
tinez-Shell Ridge fault. The position on this is established by surface 
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