Septembeb 22, 1911] 



SCIENCE 



379 



The Shinumo quadrangle is a critical area 

 for the study of the two greatest benches 

 within the canyon, the Esplanade and the 

 Tonto Platform, since it is there that the 

 profile of the canyon wall changes from that 

 which is characteristic of the Kaibab division 

 to that which is characteristic of the Kanab. 

 Eastward from Havasupai Point, a great 

 promontory of the southern wall in the center 

 of the quadrangle near Bass Camp, the 

 scenery is that which is characteristic of the 

 Kaibab division. This is the scenery that is 

 familiar to most of the visitors at the Grand 

 Canyon, from the views in the vicinity of El 

 Tovar hotel. The walls are greatly dissected, 

 particularly on the northern side of the 

 canyon. Great amphitheaters run far back 

 into the wall, filled with fantastic buttes and 

 temples, and trenched by innumerable side 

 gorges. The profile of the wall is especially 

 distinctive; the edges of the Paleozoic strata 

 descend rapidly through a series of cliffs, steep 

 slopes, and narrow ledges to the Tonto Plat- 

 form, 3,000 feet below the rim of the canyon; 

 within the Tonto Platform is the Granite 

 Gorge, at the bottom of which the river flows 

 upon Archean rocks. 



Turning westward from Havasupai Point, 

 there is a striking change. Directly below, 

 about 1,000 feet beneath the rim, a great flat- 

 topped spur of red Supai sandstone runs far 

 out into the canyon. Farther westward, more 

 and more of these spurs appear, each capped 

 with a similar platform which always lies 

 upon the same layer of red sandstone. Grad- 

 ually the platform widens and becomes a broad 

 expanse of red rock, which is covered with 

 patches of scanty soil and dotted with scrubby 

 trees of juniper and pinyon. The buttes and 

 temples disappear ; the walls are much less dis- 

 sected by side gorges and extend in solemn 

 palisades. The cross-section of the canyon 

 wall is more simple, consisting of a wide outer 

 valley whose floor is the great red platform, 

 and a narrow inner canyon, at the bottom of 

 which is the river. This platform is named 

 the Esplanade. The wall of the inner canyon 

 is stupendous, the edges of the Tonto, Eed- 



wall, and Supai strata appearing almost as a 

 single cliff, 3,000 feet in height. This type of 

 scenery is characteristic of the entire canyon 

 west of the Kaibab division. 



Opposite Havasupai Point, in the central 

 part of the quadrangle both platforms are 

 present, separated vertically by 2,000 feet. It 

 is therefore manifestly impossible that the 

 two platforms represent one base-level of 

 erosion. 



The greater dissection of the walls in the 

 Kaibab division is, of course, due to the 

 greater altitude of the plateaus on either side 

 of the canyon and the consequently greater 

 rainfall that prevails. 



The change in the profile of the wall to the 

 westward can be clearly shown to depend upon 

 certain variations in the thickness and char- 

 acter of the Paleozoic strata: Each ledge of 

 the canyon wall is made by the wasting back 

 of beds of weak strata from the summit of a 

 resistant, cliff-making stratum below. The 

 width of a ledge tends to increase with the 

 thickness of the weak strata; it is also con- 

 trolled by the relative thickness and strength 

 of the overlying strata which defend the re- 

 treat of the wall above. 



In the Kaibab division, the Bright Angel 

 shale of the middle part of the Tonto group 

 is uniformly weak and has wasted back 

 rapidly from the summit of the basal Tapeats 

 sandstone of the Tonto group, leaving the 

 wide ledge known as the Tonto Platform. As 

 the Bright Angel shale is traced westward 

 into the Shinumo quadrangle, layers of re- 

 sistant snuff-colored limestone begin to ap- 

 pear in the middle of the formation. These 

 layers, known locally as the Snuffy limestone, 

 gradually increase in thickness, making two 

 delicate parallel cliffs which are a conspicu- 

 ous feature in the interior of the canyon. 

 The Snuffy limestone increases the resistance 

 of the Bright Angel shales to erosion, so that 

 they retreat more slowly and make a steeper 

 slope; westward from Havasupai Point the 

 Tonto Platform fades gradually and is no 

 longer a prominent topographic feature in 

 the canyon. At the same time, the Muav 

 limestone of the upper part of the Tonto 



