144 
BONNEVILLE FAULTING 
hypothesis on the origin of Basin Range Structures rather than an 
elucidation of Lake matters. 
Although these delta faults were traced with numerous in¬ 
terruptions by Gilbert along the old lake beaches through a dis¬ 
tance of over 100 miles it is a singular fact that they were no¬ 
where observed to cut across any of the rock-spurs of the Wasatch 
Range, even when the latter were in direct line with adjoining 
scarps. A further curious circumstance is that some of the fault- 
scarps curved sharply so as to conform roughly with the margins 
of the deltas. Singularly, also, the expressions of fault-movement 
appear to be wholly confined to the thick delta deposits; and where 
such slipping is compound the surfaces of the several segments 
tilt away from the lake, or towards the mountain. Observed 
displacement in these delta faults is sometimes so much as 30 or 
40 feet. Were these various features the direct resultants of 
Wasatch Mountain uprising, as Gilbert urges, these idiosyncrasies 
should not prevail. Their uncalled for presence demands further 
analysis of the attendant circumstances. 
An explanation for these delta faults, other than the one which 
Gilbert advances, is to regard them as essentially landslide 
phenomena. Not the landslide of steep mountain slope, but land¬ 
slip such as characterizes unconsolidated debris masses of low or 
moderate inclination. Although the Bonneville delta phenomena 
had been repeatedly viewed personally the accepted explanation 
of the low fault-scarps was never very satisfactory. It was not 
until years after the first visit to the Great Salt Lake locality, 
after the San Francisco earthquake rifts had been examined at 
close range, near Point Reyes, that it dawned what the trouble 
really was with the Utah field. South of Point Reyes, on the hill¬ 
sides some little distance from the great fault-line, the earth was 
creviced, or “faulted,” but it was a slipping of the hillside soil 
masses. Similar phenomena were often presented when in rail¬ 
way cuts the local watertable was encountered. 
Applying the principle of the tapping of the local watertable 
to the Bonneville deltas the necessary consequences exactly tallied 
with the observed features. In the case of the old deltas the huge 
spongy masses were held in tact by the high lake waters, and re¬ 
mained in undisturbed repose so long as the great water body 
pressed upon and against them. With the dropping of the lake 
