The Shinumo Area. 501 



The total areal exposure of the Grand Canyon series in the 

 Shinumo area is about 12 square miles. 



In following up the Shinumo from its mouth to the point 

 where it leaves the lateral gorge of the Muav-Flint Creek can- 

 yon, a traverse is made of the total exposed thickness of the 

 Unkar group from the unconformity at the base to the highest 

 member that is limited by the profound fault on the northeast. 

 There can hardly be a more magnificent illustration of details 

 of geological structure than is here revealed. Along the entire 

 western side of the Shinumo canyon, in cliff faces a thousand 

 feet above the bed of the stream, is displayed every detail of 

 the structure beneath the basal Tonto sandstone. Westward 

 down the Colorado river the intersection of the two great 

 un conformities forming the apex of the wedge is seen in the 

 cliff face above the river bank, below which point the river 

 narrows in its somber gorge in the Yishnu schists. From here 

 northeastward, in the cliff faces along the western wall of the 

 Shinumo canyon, bed after bed of the Unkar strata appears, 

 wedging out southwestward beneath the plane of the uncon- 

 formity beneath the Tonto sandstone. Every detail of the 

 successive fault blocks of the great wedge is clearly shown, — 

 their increasing tilt northeastward, the dips of the fault planes 

 that bound them, and the occasional down-dropped w T edges. 

 Above runs the plane of the pre-Tonto unconformity, revealing 

 in cross section the monadnock in this peneplain which existed 

 as a rocky island during the inroads of the Tonto sea, the debris 

 from its wave-cut cliffs being incorporated and preserved to the 

 minutest detail in the Tonto sandstone. In the background, 

 bed above bed in conformable succession, lies the horizontal 

 Paleozoic section in the wall of the mile-deep Canyon. After 

 traversing a thickness of 5800 feet of Unkar strata in a distance 

 of three miles dipping northeastward into the bed of the stream, 

 the traveler crosses the line of the great pre-Cambrian fault of 

 more than 5800 feet and comes once more into the Yishnu 

 schists on the farther side of the Muav-Flint Creek canyon. 



Here again is an instance of the simplicity with which the 

 geological structure is revealed in this wonderful country. 

 Along the whole northern wall of the lateral gorge lie the 

 Yishnu schists below the Tonto sandstone cliff. On the south- 

 ern side, at the same level, lie the upper sandstones of the 

 Unkar, their beds dragged up sharply against the fault line, 

 which lies in the bed of the stream. The whole Paleozoic 

 system on the northern side of the gorge has been dropped 

 500 feet by the torn monocline of the West-Kaibab fault, 

 reversing the throw of the pre-Cambrian fault on the same 

 line in the basement rocks. Looking westward up the Muav 

 Canyon, the beds of the Paleozoic are seen bending down 



