DAVIS: THE GRAND CANYON OF THE COLORADO. 121 
from the southern rim at Cameron and Berry’s, that the upper Aubrey 
cliffs which descend into the canyon from a detached part of the 
Kaibab plateau that lies south of the canyon are notched by wide-open 
mature valleys which descend eastward with the monoclinal flexure that 
terminates the plateau in that direction. The valleys evidently once 
had a greater extension headward, but they have now lost some of their 
upper length by the widening of the steep-walled canyon; thus recalling 
the relation existing between the mature centrifugal valleys of Mt. 
Mazama and the sharp cliffs of the caldera that holds Crater lake in 
southern Oregon. It is normal enough for the deep-cut canyon to 
encroach upon the heads of valleys that descend’ away from it; but 
if the upper Aubrey had been resistant enough to hold the canyon 
narrow while the Triassic sandstones were worn away for scores of miles 
on either side, we should hardly expect to find beheaded valleys of a 
mature form; yet such a relation would be easily explained on the sup- 
position that the beheaded valleys still retain, little changed, a mature 
form that they acquired in an earlier cycle of erosion, while the main 
river has destroyed its mature valley of the earlier cycle by incising the 
canyon in its floor. 
Tue LANDSLIDES OF VERMILION AND Ecuo Cuirrs.—If the erosion 
of the plateaus and the canyon had been accomplished in a single cycle, 
there should be at the present advanced stage in the cycle no signs of 
revival in the processes of denudation on the retreating cliffs of the 
plateaus. If, on the other hand, there have been two cycles, revivals 
should be of frequent occurrence and of systematic distribution. The 
wonderful series of landslides that occur along the base of the Vermilion 
and Echo cliffs, on the western and eastern sides of the great notch in 
the Triassic escarpment that heads up at Lee’s Ferry, seem to be features 
of this kind. It is noticeable that these slides occur relatively near the 
river, while further away the cliffs have a much more mature profile, 
entirely without slides. Inasmuch as landslides are characteristic of 
the earlier and more energetic stages of a cycle of erosion, it seems 
probable that those here described should be associated rather with the 
reviving activities in the youthful stage of a second cycle than with the 
fading activities in the advanced stage of a first and single cycle. 
The neighborhood of Lee’s Ferry is especially interesting in this con- 
nection. Here the Carboniferous strata of the Marble platform? dip 
1 It seems permissible in the interest of brevity to speak of this plateau as the 
“Marble platform,” instead of as the “Marble canyon platform.” In the same 
way, the Echo cliffs monocline will be called the Echo monocline. 
VOL. XXXVIII. — NO. 4 2 
