GEOLOGIC HISTORY. 57 
PALEOZOIC AND MESOZOIC CONDITIONS. 
But little can be said of theJiistory of this region during Paleozoic 
and Mesozoic time. Cambrian sediments, consisting of sandstone 
and shale (Tonto formation), underlie the plateau, and were prob- 
ably deposited over a large part of western Arizona. No Ordovician 
or Silurian formations have been discovered, and the Devonian of 
Grand Canyon, described by Walcott, a was not identified in the area 
examined. The Carboniferous is represented by the Redwall lime- 
stone, which extends from the Mississippian into the Pennsylvanian, 
as previously shown in the description of rock formations (pp. 15-16). 
There is a limited exposure of Carboniferous strata younger than the 
Redwall in Iceberg Canyon, and Mesozoic formations occur east^of the 
rim of the plateau in a manner which suggests the probability that 
they originally extended westward over the region described. So far 
as known, however, no remnants of any of these formations remain 
west of the cliffs. 
Little is known regarding the original extent of the older sedimen- 
tary formations in western Arizona or of the circumstances attend- 
ing their removal. Probably some of the erosion occurred during 
the Cretaceous period, as suggested by Gilbert, 6 but nothing was 
found to indicate what proportion of it was accomplished in pre-Ter- 
tiary and what in early Tertiary time. 
TERTIARY EVENTS. 
Explanatory statement. — No great amount of sediment was depos- 
ited within the area described during Tertiary time. The period was 
apparently one of uplift, volcanic activity, and extensive erosion, but 
it is not known whether the uplift began with the Tertiary or before, 
nor can the Tertiary be confidently separated from the Quaternary. 
The sequence of events, based upon physiographic evidence, can be 
stated with some confidence, but the various epochs can not be confi- 
dently referred to established subdivisions of time. 
Denudation. — The erosion which removed the older sedimentary 
formations from western Arizona during early Tertiary and pre- 
Tertiary time was evidently on an extensive scale. Whatever Paleo- 
zoic and Mesozoic sediments had been deposited were stripped off 
and the underlying granites deeply dissected previous to the erup- 
tion of the oldest effusives. The Eocene sediments extensively devel- 
oped farther north in parts of the plateau region are apparently absent 
from western Arizona, and the extensive denudation may have been 
accomplished in large part during Eocene time. 
« Walcott, C D., Am. Jour. Sei., 3d ser., vol. 20, 1880, pp. 221-225. 
. *>Gilbert, (i. K., U. S. Geog. Surv. W. 100th Mor., vol. 3, 1875, pi. I. p. L34. 
