oie. 3 
THE SANTA FE ROUTE. 125 
constitute the middle slopes of the canyon walls, usually presenting a 
great series of terrace-like steps of red sandstone. These steps are 
caused by the projection of harder layers of sandstone. The Redwall 
limestone * forms a conspicuous cliff at the foot of the Supai slopes. 
The rock is hard and massive, and its resistance to erosion makes 
it a prominent feature in the canyon. Its surface is stained red by 
wash and drippings from the overlying red shales. 
The Redwall and the overlying Supai, Coconino, and Kaibab beds 
represent the greater part of the Carboniferous period. (See p. ii.) 
The Supai, Coconino, and Kaibab are of about the same age as the 
limestones along the Santa Fe line from Kansas City to Strong City, 
Kans., but there is a marked difference in their character. 
The Tonto group, below the Redwall, consists of 800 feet of shales, 
largely of greenish color, and a basal sandstone averaging 150 feet 
in thickness. This group is very much older than the Redwall, and 
though at their contact the beds of the one are practically parallel to 
the beds of the other, there is a hiatus here which represents a very 
considerable portion of geologic time not represented by rocks in 
this region but recorded by many thousand feet of rocks in other 
portions of North America and in other countries. The shales make 
a long slope, interrupted by some subordinate ledges of limestone 
and sandstone, descending to a pronounced shelf of the sandstone, 
called the Tonto Platform. This slope and the wide shelf at its foot 
are both very characteristic and easily recognized features extend- 
ing along the lower slopes of the Grand Canyon. 
For many miles this shelf of sandstone of the Tonto group 1s cut 
through by the steep inner gorge (shown in Pl. XX XIII, p. 127), which 
descends to the river, 800 to 1,000 feet below, and exposes the under- 
. lying granite and gneiss in very dark rugged ledges. These rocks 
are part of the old earth crust, which has been subjected to great heat 
and pressure. Later in its history its surface was worn down to a 
plane upon which were deposited thick beds of sand, clay, and other 
materials. In a wide area the basal sandstone of the Tonto lies 
directly on the smooth surface of this schist and granite, but in some 
places, notably in the broad part of the canyon northeast of Grand- 
view, in Shinumo basin, in part of Bright Angel Canyon, in Ottoman 
and Hindu amphitheaters, and in the ridges extending northwest and 
southeast from a point near the mouth of Bright Angel Creek other 
rocks lie between the granite and the Tonto rocks. These are 6 
succession known as Grand Canyon series, comprising the Unkar 
oe ee ee 
*The Redwall limestone projects in | sion, such as Cheops Temple, Newberry 
many flat-topped spurs and buttresses | Butte, and Sheba Temple, which form 
and constitutes outliers isolated by ero- | striking topographic features. 
7 97579°—Bull. 613—15——_9 ., 
