DAVIS: THE GRAND CANYON OF THE COLOEADO. 121 



from the southern rirn 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 head ward, 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. 



The Landslides of Vermilion and Echo Cliffs. — If the erosion 

 of the plateaus and the canyon had been accomplished in a single cvcle, 

 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 1 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 



