374 Noble — Geology of the Grand Canyon, Arizona. 



Topography, Climate, and Vegetation. 



The surfaces of the plateaus in the Shinumo area through 

 which the pathway of the Grand Canyon is trenched are every- 

 where developed at the same horizon on the highest member 

 of the Paleozoic series occurring in the Canyon wall and known 

 as the Kaibab limestone, the Mesozoic and Tertiary formations 

 having been stripped back to the terraces of southern Utah. 



The surface of the southern, or Coconino, plateau slopes to 

 the southwest away from the canyon rim at the rate of about 

 200 feet to the mile. The drainage system of the plateau 

 surface consists of a series of mature, open-floored valleys with 

 gently sloping sides, which contain no living streams. These 

 valleys trend southwesterly away from the canyon rim with 

 the slope of the plateau surface and drain into the broad, shal- 

 low synclinal basin occupied by Cataract Canyon. In tracing 

 one of these mature valleys toward its head, it is a common 

 thing to find it truncated as a hanging valley by the wall of 

 the Grand Canyon. So general is this phenomenon, that the 

 stranger who loses his way on the southern plateau has only to 

 keep in mind that if he follow any valley far enough headward 

 he will come out upon the rim of the Grand Canyon. 



The surface of the northern, or Kaibab, plateau is in every 

 respect similar to that of the Coconino. The same system of 

 mature valleys covers its surface, which slopes southwesterly to 

 the rim of the canyon. The only difference is that the surface 

 drainage runs into the Grand Canyon instead of away from it. 

 Neither plateau surface contains a living stream. 



Powell Plateau may be regarded as a dismembered part of 

 the Kaibab. Its surface is developed at the same horizon on 

 the Kaibab limestone, slopes southwesterly at the rate of 900 

 feet in five miles, and contains the same mature drainage 

 system. It is substantially a great island, surrounded on three 

 sides by mile-deep canyons, and isolated from the mainland by 

 erosion in the line of displacement of the West Kaibab fault. 

 The gap that separates Powell Plateau from the mainland is 

 called the Muav Saddle, and is notched 800 feet below the 

 surface of the plateaus. No more striking topographic con- 

 trast can be imagined than that between the mature drainage 

 systems of the plateaus and the youthful topography within 

 the deep-trenched canyon. 



The strata of the Paleozoic rock system within the area dip 

 gently and almost uniformly to the southwest at the rate of 

 about 200 feet to the mile, and the surfaces of the plateaus are 

 everywhere accordant with the rock structure. A slight local 

 interruption occurs where the line of the "West Kaibab fault 

 crosses the area, along which the strata are flexed into a mono- 



