COTTONWOOD VALLEY—STRATIFIED GRAVELS. 33 
Similar deposits would doubtless have been found on the sides of the other cafions of the 
Colorado; but such accumulations have generally, from the greater height of the barrier, been 
less in quantity, longer exposed to atmospheric degradation, and, as a consequence, have 
disappeared. 
COTTONWOOD VALLEY. 
For some miles above Pyramid cafion the river flows through an area which, though occupying 
the space between mountains on the east and west, scarcely deserves the name of valley. 
Granitic masses connected with the Dead mountain form the immediate or remote bank on the 
west side, while on the east a succession of gravel terraces rise abruptly from the water’s edge, 
and extend in table-topped hills to the base of the Black mountains. 
These hills have been eroded from the mass of the terraces by the water which sometimes 
flows down through them from the summits of the mountains. The material which composes 
the gravel beds is usually considerably indurated, and cut, as they are, by water flowing from a 
distance, their erosion has assumed the form of a labyrinth of narrow ravines or cafions, of which 
the opposite perpendicular walls, though often one hundred feet in height, approach each other 
so closely that they may be touched at the same time by the outstretched hands. 
In many localities the erosions of these Quaternary beds—for of this age I must regard them— 
have exposed underlying masses of porphyry, trap, or tufa similar to those visible in the higher 
parts of the Black mountains. Upon the irregular surface of these rocks the consolidated gravels 
are deposited in nearly horizontal stratification. 
A section of the table-hills in this vicinity is as follows: The summit rock is a cream-colored 
limestone, six feet in thickness, nearly pure above, but becoming a breccia below. The struc- 
ture of this limestone is waved, like that of ripple-marked clays or sandstones, and its upper 
surface is most curiously weathered, exhibiting on a great scale something of the same style of 
erosion as that of the blocks and pebbles of limestone collected on the desert. Usually it pre- 
sents a naked surface, without soil, upon which, here and there, rest large boulders of black 
basalt or scoria. Of the few rolled pebbles observed upon it, some were of Carboniferous lime- 
stone, and had evidently been transported from a great distance. I observed no fossils in this 
rock, but there is little doubt that it is of lacustrine origin. 
Below the limestone is a succession of strata of indurated gravel, or conglomerate beds of 
coarse or fine, rounded or angular, fragments of rock, enveloped in sand and cemented by lime- 
There is much less clay in this series here than in any similar deposits observed below, showing 
that the currents which deposited them were comparatively rapid. The variety in the compo- 
sition of the bars in the present river in different parts of its course is dependent on the 
Same cause. Here they are of gravel; lower down, of sand, and at the mouth, of clay. 
The boulders and angular fragments included in these conglomerates exhibit great diversity 
of composition, and have been derived from widely different sources. They may be classed as 
follows : 
Firet. Angular blocks of all sizes of porphyry in endless variety, granite, sienite, trap, 
Scoria, trachyte, &c., brought down formerly, as now, from the adjacent mountains. 
Second. Smaller pened boulders and pebbles of limestone, generally fossiliferous, but some- 
times metamorphic. The most common is a blue unchanged limestone, of Carboniferous age, 
containing crinoidal columns of large size, Productus, Spirifer, and Pecten, with Fenestella and 
many corals. These, as our subsequent explorations demonstrated, were derived from the 
Carboniferous limestones of the Great Cajion of the Col lorado ; the limestone boulders without 
fossils representing the strata which underlie the Cuskeaiiens series. - 
ird. Rolled pebbles of quartz, jasper, silicious slates, granite, carnelian, &c., which have 
been transported from a great distance. Many of these are derived from the Black cafion, 
especially the quartz and the granite with garnets. The jasper and carnelian are, principally, 
secret 
