752 H. H. ROBINSON 



of this plateau is a stripped structural one formed by the upper 

 Aubrey cherty limestone, and its elevation above the surrounding 

 plateau country is due principally to monoclinal folding which 

 occurred not later than the very beginning of the erosional history 

 of the region in the Eocene. It must be supposed, then, that the 

 surface of the Kaibab Plateau was reduced to maturity or old age, 

 coincident with the development of the peneplain, and that its 

 present mature topography represents a continuation of the process 

 in the post-peneplain cycle of erosion. 



The separation of the post-peneplain cycle from the canyon 

 cycle which followed is based in part on the wide difference in the 

 character of their drainage systems, the one perfect in its maturity, 

 the other equally perfect in its youthfulness, and the fact that 

 the canyon cutting has to some extent destroyed the mature 

 valleys. There is abundant evidence that this latter process is 

 still in progress ; it is very plainly seen on both the north and south 

 sides of the Kaibab section of the Grand Canyon where the older 

 mature valleys are being consumed by the widening of the youth- 

 ful canyon. Further, the courses of many of the partly consumed 

 valleys lead away from the canyon, thus indicating that there has 

 been such a radical readjustment of drainage lines in the canyon 

 cycle as to bring about definite changes in their direction, and 

 this is especially noticeable in the case of the trunk stream of the 

 region — the Colorado River. 



The tracing out of the mature valleys and reconstruction of the 

 drainage system of the post-peneplain cycle of erosion is evidently 

 an important problem which must be worked out before a satis- 

 factory explanation for the location of the present canyon system 

 of drainage can be offered. For the reconstruction of this mature 

 drainage system should permit a definite idea of the attitude of 

 the land during the post-peneplain cycle to be formed and con- 

 sequently indicate the extent and magnitude of the regional warp- 

 ing which followed. Of equal importance, also, would be a study 

 which would permit the reconstruction over an extensive area of 

 the peneplain of the preceding cycle of erosion; this would assist 

 in determining not only the extent of the warping but also the 

 magnitude of the faulting which has occurred at various times in 



