126 Robinson — Tertiary Peneplain of the Plateau District. 



depends on their common relation to the Plateau structure. 

 The Aubrey Cliff, which crosses Arizona in a northwest direc- 

 tion, here intersects the Colorado. Since the general dip of 

 the strata is to the north, and the escarpment is due merely to 

 their unequal denudation, there lie, at the foot of the escarp- 

 ment, a series of monoclinal valleys, of which the Tonto Basin, 

 the Upper Verde Valley, and Aubrey Valley are examples. 

 Diamond Creek runs, in like manner, parallel to the cliff, and 

 differs from the others only in having excavated a deep gorge, 

 which its low level of discharge enabled it to do. The same 

 Aubrey Cliff that follows its northern and northeastern margin 

 reappears beyond the Colorado, and, for forty miles, bears the 

 same relation to the lower course of the Grand Canyon, leading 

 to the belief that the stream was here guided, at the first, by 

 the monoclinal valley, and that the Aubrey Cliff, as a topographic 

 feature, is more ancient than the Grand Canyon. The cliff 

 now rises from three to five miles back from the brink of the 

 canyon, and may be supposed to have retired to that position 

 by slow waste during the excavation of canyon." 



Gilbert's observation in regard to the course of the Colorado 

 between Diamond Creek and the Uinkaret Plateau — that it 

 coincides with a fault — has been verified by the later work of 

 Dutton* on the Hurricane fault. While the river from Diamond 

 Creek to the Grand Wash, in the light of present knowledge, 

 should be regarded as adjusted to the structure underlying the 

 peneplain rather than guided by a preexisting valley. The 

 description is very clearly that of a subsequent stream, and as 

 it applies to one-third of the course of the Colorado in the 

 Grand Canyon District, its bearing on the origin' of the river is 

 evident. 



If the boundary between the peneplain and lowlands in the 

 Little Colorado Valley lay not far to the east of the river, as 

 the observations at Black Point seem to show, then, when the 

 uplift and tilting occurred, a northwestward sloping depres- 

 sion would be formed along the boundary. The waters of the 

 streams on both slopes of this .depression would be gathered 

 along its line of lowest level into a trunk stream, which would 

 flow northwestward until the unfavorable southward slope at 

 the northern boundary of the peneplain was met. At that 

 point it would be turned sharply south, joining the westerly 

 flowing consequent that would there be developed. Or, if the 

 peneplain extended still farther east than is supposed, the 

 Little Colorado may be even more simply explained as a con- 

 sequent that developed on the uplifted peneplain, and is now 

 strictly governed by the underlying rock structure. As the 

 dip of the strata is in general northeastward, the river in 



*Op. cit. (a), chap. vi. 



