Robinson — Tertiary Peneplain of the Plateau District. 125 



of the Kaibab until the waters rose to the highest point on the 

 boundary at the southern end of the plateau, when overflowing 

 this point they would continue northwestward. What may 

 have determined the southwestern course from Kanab Creek 

 to the Uinkaret Plateau is uncertain, although it may have been 

 influenced by the lava flows now represented by the eroded 

 remnants in Mounts Emma, Logan and Trumbull. At the west 

 side of the Uinkaret Plateau the river turns southwest and 

 then south to Diamond Creek, where it changes its course 

 abruptly to the northwest, which direction it holds until it 

 reaches the Grand Wash and passes beyond the border of the 

 plateau. The course of the river was observed by Gilbert* 

 in 1871 from Diamond Creek, and his description may be here 

 quoted : 



" At the mouth of Diamond Creek center four valleys of 

 denudation, whose relation to the structure of the Plateau is 

 very interesting. Two of these valleys are occupied by the Colo- 

 rado and are portions of the Grand Canyon ; that is to say, the 

 river turns here abruptly at a right angle, and its two courses 

 appear to have been determined by distinct causes. The third 

 valley is the canyon cut by Diamond Creek, and is a prolonga- 

 tion eastward of the lower course of the Colorado. The fourth is 

 that of Peach Spring Wash, and prolongs southward the upper 

 course of the Colorado .... The character and remarkable 

 straightness (of Peach Spring Wash) — I refer to its general 

 course, and not to its details — are due to the fact that it was 

 primarily determined by a fault ; ... its throw is to the west, 

 and the amount of its dislocation near the river is about 600 

 feet. Its strike is north 25° east. For a few miles at least it 

 is included in the upper course of the Grand Canyon, and, as 

 I looked down Peach Spring Wash, and commanded with my 

 eye a long vista of the canyon beyond, I was strongly impressed 

 with the idea that the dislocation that had determined the one, 

 had also marked out the other for a long distance. Later geo- 

 graphical determinations show that from the neighborhood of 

 the Uinkaret Mountains to the mouth of Diamond Creek, a 

 distance of thirty miles, the general direction of the canyon is 

 straight and coincident with the observed trend of the fault. 

 If what now appears probable shall hereafter be demonstrated 

 — that the canyon for a long distance follows closely the line 

 of faulting — the necessary deduction that the fault antedates 

 the beginning of the canyon will be an interesting addition to 

 the cronology of the river. 



" The identity in direction of Diamond Creek with the lower 

 course of the Grand Canyon is not a mere coincidence, but 



-Op. cit., p. 79. See also Mt. Trumball and Diamond Creek topographic 

 sheets, U. S. G. S. 



Am. Jour. Sci. — Fourth Series, Vol. XX [V, No. 140.— August, 1907. 



