DAVIS: THE PLATEAU PROVINCE OF UTAH AND ARIZONA. 41 
was eroded. Some account of the older topography will be given in the 
Bulletin by Messrs. Huntington and Goldthwait to be issued. 
The huge landslides along the base of Echo and Vermilion cliffs east of 
the Kaibab were ascribed, in my previous essay, to an active sapping 
of the weak Triassic clays under the cliffs in consequence of the revival 
of erosive processes that followed the uplift of the Canyon cycle. 
Numerous landslides of similar stratigraphic position were observed 
during the past summer in the valley of the Virgin near Rockville. 
They are extensive and abundant, and large enough to appear in Plates 
6 Aand 7A. It is a matter of surprise that they are not mentioned in 
Dutton’s account of this district. Where the valley is broad, the 
slides lie for the most part on the Shinarump bench, as in Figure 14 ; 
they have the form of disorderly mounds and hills, from two hundred 
to six hundred feet high, strewn with huge boulders for which there is 
no local source in the form of immediately surmounting cliffs ; indeed, 
some of the mounds stand one or two miles forward of the Triassic cliffs 
from which they were derived. One example was noted in which the 
front of a slide descended below the Shinarump cliff into a ravine worn 
in the Permian slopes beneath ; and this slide has been utilized. for the 
construction of the roadway from Canaan spring to Rockville, the 
Shinarump cliff being apparently impassable elsewhere for several miles 
up and down the valley. There appears to have been a significant 
amount of erosion since the slides took place, for their slopes seemed to 
be of greater uniformity than would likely have been the case in their 
initial disorder. . 
As one passes up the valley of the North fork, the Shinarump bench, 
Plate 7 A, gradually decreases in height and soon disappears under the 
valley floor. The landslides then advance directly toward the stream, 
and five miles further north they form a serious barrier in its course : 
here for a mile or more the channel is interrupted by bouldery rapids. 
The landslides then cease, because the weak clays by whose sapping 
they were caused have run under ground, and the valley floor has a flat 
flood-plain for two miles or more, apparently the result of alluvial filling 
caused by the landslides barrier below. The flood-plain is known as 
Zion ; it is cultivated by the farmers who come up the rough road over 
the landslides from the villages down the valley. A mile further on, 
the stream bed rises above the shaly sandstones that elsewhere form a 
slope beneath the heavy upper Trias cliff-maker, and here the cliffs close 
in precipitously upon the stream, producing the cleftlike Mukuntuweap 
‘canyon, already mentioned. The rock walls of Zion, Plate 6 B, are 
