Robinson — Tertiary Peneplain of the Plateau District. 127 



either case would occupy a inonoclinal valley, like the Colorado 

 from Diamond Creek to the Grand Wash, the eastern wall of 

 which would be well marked where resistant strata were 

 encountered, or indistinct where they were soft. 



The actual conditions appear to agree closely with the 

 inferred. The profile of the river is quite unique in that it is 

 a distinctly reversed curve, the upper and lower ends being 

 much steeper than the intermediate portion. This may be 

 illustrated by the following figures : For the first 40 miles, 

 measured from the mouth of the river, the average grade is 

 34 feet per mile, for the last 40 miles it is 55 feet, while for 

 the intervening stretch of 200 miles the grade is but 9 feet per 

 mile. In more detail the grades of the first 200 miles are : 

 (a) 25 miles at 35 feet per mile, (b) 15 miles at 31 feet, (c) 35 

 miles at 15 feet, (d) 100 miles at 7'5 feet, (e) 25 miles at 18 

 feet, with the remaining 80 miles at increasingly steeper grades. 

 The two points to notice are the steep grade for the first 25 

 miles (a), and the low grade for the 100-mile stretch (d). The 

 first shows the effectiveness of the heavy Carboniferous forma- 

 tions, in which the river has cut a precipitously walled can- 

 yon, in preventing a normal grade from being developed in the 

 lower course of the stream ; the second illustrates the same 

 point, since it has resulted from the river being obstructed by 

 the same formations in the vicinity of the "Crossing" north 

 of Canyon Diablo. The river is thus closely controlled by the 

 strata it flows upon, and is far from uniformly graded. The 

 character of the river, near Winslow, in section (d) has been 

 described by Marvine.* He said : " At this point, and above, 

 the river is not at all typical of this region, inasmuch as it 

 does not flow in a canyon. On the contrary, its flood plain is, 

 in places, over a mile or more in width, through which it 

 winds its muddy current in quite a tortuous course. It prac- 

 tically occupies a monoclinal valley between the Carbonifer- 

 ous and Triassic." The river continues in this monoclinal 

 valley until the deep canyon at its mouth is reached, and of its 

 course in the vicinity of Tanner's Crossing, Davisf has said 

 that " It may be plausibly regarded as a subsequent stream ; 

 such was certainly its habit where we crossed it." 



The river in its course northward passes through the Shina- 

 rump and Moencopie formations, and especially in its lower 

 course, though also between Grand and Black Falls, through 

 the Carboniferous. The western slope of the valley is clearly 

 a stripped structural surface, and, except near the river, is 

 marked by the Upper Aubrey cherty limestone. The north 

 side of the valley is limited by a series of prominent cliffs, 

 *Op. cit., p. 214. fOp. cit. (a), p. 153. 



