136. GUIDEBOOK OF THE WESTERN UNITED STATES. 
"are wellexposed. Their thickness is 12,000 feet or more and the beds 
dip at moderately steep angles. Their surface has also been worn 
off to a rolling plain, with many local hills on which le the shales of 
the Tonto group. The Unkar group, which is the one exposed from 
most points of view, consists of a succession of basal conglomerates, 
dark limestone in thick beds, bright-red shales, heavy quartzites, and 
brown sandstones.! 
This succession of rocks is plainly visible in 
Bright Angel Canyon and in the ridge culminating in Cheops Pyramid 
(see Pl. XXX), also in a wide area along the river in the region 
northeast of Grandview Point. 
Many: interesting features of the geologic history of the plateau 
region are recorded in the rocks of the Grand Canyon, and a summary 
of these records is given below.” 
! These rocks have been named Hotauta 
conglomerate, Bass limestone, Hakatai 
shale, Shinumo quartzite, and Dox sand- 
stone. 
2 The granite and gneiss at the bottom 
of the canyon are part of the oldest group 
of nae constituting the earth’s crust. 
eiss, which is the older, isin nearly 
erat layers. It has been subjected to 
great heat and pressure, and into it the 
ite was forced in a molten 
Later the surface of these rocks was eroded 
to a plain by running water. 
The next event = which there is evi- 
dence was the submergence of this plain 
and the deposition in water, of varying 
depth, of a thick series of sediments now 
represented by the 12,000 feet or more of 
sandstone, limestone, and shale consti- 
tuting the Unkar and Chuar 
‘hese strata are believed to represent the 
Algonkian period (see p. ii), the earliest 
in which remains ot life have been found. 
accum : 
materials of the limestone were laid 
Next there was extensive uplifting of 
the earth’s crust, with tilting and faulting 
indicated by the relations of the rocks on 
the north side of the river opposite El 
Tovar 
When the surface was reduced to a roll- 
ing plain with a few hills rising in places, 
there was another submergence by the 
sea, which deposited the sediments of 
the Tonto group. First the sand was de- 
posited over the smooth granite surface 
(as shown by the heavy line in fig. 28, B). 
With deepening waters or diminishing 
force of the currents, the clay now repre- 
nted by the shale of the Tonto group 
was laid down, soon burying the islands 
of Unkar and Chuar rocks and accumu- 
lating to a thickness of 800 feet or more. 
Remains of life in these rocks indicate 
region during the next three long and 
very important geologic eee are not 
known, for their representatives are ab- 
sent except a small amount of oe De- 
vonian rocks found at one or two places. 
The sea may have laid down here, during 
those periods, deposits of great thickness, 
which were later uplifted into land areas, 
so that they were removed by streams 
and other agents of erosion. 
In early Carboniferous time, the period 
Sseetpgaeeas in other parts of the world 
y the accumulation of the older coal- 
ee deposits, the entire region was 
submerged by the sea, which deposited 
calcium carbonate in nearly pure condi- 
tion, now represented by 500 feet or more 
of the Redwall limestone. Much calcium 
is carried into the sea by streams, and its 
