DAVIS: THE GRAND CANYON OF THE COLORADO. 141 



cline without fracture, and has concluded from this that a considerable 

 load of overlying strata must have covered the Kaibab when the flexure 

 was produced (d, pp. 60, 64) ; in other words, that the flexure occurred 

 sometime before the close of the plateau cycle. 



If this conclusion be correct, the crown of the Kaibab arch must 

 have risen a thousand feet or more above a denuded lowland on the 

 east and west at the close of the plateau cycle. The strikingly even 

 skyline of the Kaibab, as seen from any distant point, is therefore to be 

 regarded as indicating a stripped structural (upper Aubrey) plain, and 

 not as a part of a once baselevelled surface, afterwards uplifted. The 

 exposure of the arch to denudation during the later stages of the 

 plateau cycle, as well as through so much of the canyon cycle as has yet 

 elapsed, is indicated by the much greater dissection of its surface than 

 that of the Kanab, which, as explained above, is best regarded as a 

 stripped structural plain of the current cycle. The difference between 

 the greater and less dissection of the two plateaus is indicated in 

 Dutton's descriptions, for one is there spoken of as diversified by valleys 

 which " cover the entire surface " (c, p. 134), while the other is said to 

 be (except for deep canyons) " no more uneven than the rolling prairie 

 of Iowa " (c, p. 124) ; and this contrast of form may be easily recognized 

 on comparing sheets VIII. and XI. of the Grand Canon Atlas. Another 

 corollary of the early uplift of the Kaibab arch is that an open valley 

 must have been eroded across it in the later stages of the plateau cycle. 

 If this be true, then it must be further supposed that the rapid widen- 

 ing of the new and deep cauyon in the current cycle has consumed all 

 traces of the earlier valley. 



The Crags of Echo Cliffs. — A peculiar feature of the Echo cliffs 

 deserves brief mention. In looking along this great escarpment from 

 the south, its crest at several points between Tuba and Lee's Ferry is 

 seen to rise in serrated peaks with sharp slopes on the east as well as on 

 the west, thus departing from its ordinary tabular form. We at first 

 took the sharp serrations to indicate a local increase of dip, and on 

 coming abreast of them were much surprised to find the dip practically 

 unchanged from its uniformly moderate measure. The front view of 

 the escarpment at certain points between the serrated peaks disclosed 

 sags in the crest line that could not be explained by any local thinning 

 of the cliff-making sandstones, for they seemed to maintain sub- 

 stantially a uniform thickness for tens of miles together. It was there- 

 fore suggested that the peaks and sags resulted from the occurrence of 

 strike faults with small throw, torn on the slope of the monocline. As 



