500 Noble — Geology of the Grand Canyon, Arizona. 



the deposition of the entire Paleozoic series of the Canyon 

 wall and probably later strata. The first of these is a inono- 

 clinal flexure which reverses the throw of the pre-Cambrian 

 fault, while the second is a still more recent fault super- 

 imposed upon the line of the monoclinal flexure, 



The strata of the Grand Canyon series are exposed beneath 

 the Tonto sandstone in all that part of the inner gorge of the 

 Mnav-Flint Creek canyon which is on the south side of the great 

 pre-Cambrian fault, — a distance of about three miles. They 

 are exposed for three miles in the gorge of the Shinumo 

 Canyon; for seven miles along the north side of the Colorado 

 river ; and in all the inter-canyon valleys within that distance 

 which are eroded below the base of the Tonto sandstone. The 

 two largest of these inter-canyon valleys are the " East Wash," 

 a mile east of the Shinumo, and the " Asbestos Canyon," three 

 miles to the west. 



The gorge of the Colorado river has everywhere been 

 trenched to a depth sufficient to expose the Vishnu schists 

 along the river beneath the overlying strata of the Grand 

 Canyon series. This is due to the fact that the course of the 

 river lies close along the southern apex of the wedge. 



The exposures on the south side of the river are more lim- 

 ited, due to the thinning out of the wedge in that direction 

 and the lack of inter-canyon valleys trenched beneath the Tonto 

 sandstone. The strata are exposed for two miles above and 

 one mile below a point opposite the mouth of the Shinumo. 



A southwestward bend in the river in the western part of 

 the area carries it beyond the apex of the wedge, below which 

 point the Tonto sandstone caps the Yishnu schists which lie in 

 the river gorge. Southeastward up the river, in the eastern 

 part of the area, a similar relation obtains. 



The hard middle members of the Unkar resisted the erosion 

 which preceded the deposition of the Cambrian sandstone and 

 stood as an island in the Tonto sea. 



This long monadnock of quartzite runs across the area in a 

 N.W.-S.E. direction parallel to the general strike of the strata 

 of the wedge, and a narrow outcrop of these quartzites is ex- 

 posed along the Tonto platform on the north side of the river 

 just at the base of the Redwall cliff, running for a distance of 

 about five miles beyond the main areal exposures about the 

 mouth of the Shinumo and uniting with the limestones of the 

 upper Tonto group to form the lower part of the great cliff of 

 Redwall limestone. In the eastern part of this exposure the 

 quartzite monadnock projects 700 feet above the base of the 

 Tonto sandstone. Westward from the exposures about the 

 Shinumo the prolongation of the monadnock along the strike 

 of the quartzites exposes them in a narrow outcrop upon the 

 Tonto platform one mile west of the Shinumo. 



