Reports of explorations and surveys, to ascertain the most practicable and economical route for a railroad from the Mississippi River to the Pacific Ocean: Vol. 5, Pt. 2

SURFACE OF THE COLORADO DESERT.
prevailing trend of the granitic elevations to which the region owes its configuration. A predominant northwest and southeast trend of the principal lines of elevation is said to characterize
the mountain ranges along the Gila River. 1 It is also visible, in detail, in the range on the
north of the Desert, where the ridges project in a series of overlapping points, forming long and
re-entering angles. This appears to result from a series of parallel ridges succeeding each other
from west to east, and overlapping towards the southeast. This composite character will be
found in nearly all of the mountain ranges of California; but, so far as my observations have
extended, this is the only instance where the overlapping of the ridges is towards the southeast
it is generally the reverse, or towards the northeast.
ASPECT OF THE SURFACE OF THE DESERT.
Before I reached the surface of the Desert I had been accustomed to regard it as a vast plain
of gravel and sand, and supposed that the latter was so deep as to impede the progress of
animals and wagons. This, I believe, corresponds with the general impression regarding the
Desert. Instead, however, of the whole plain being composed of loose and sandy materials, a
great part of it is formed of a compact, blue clay, which has a smooth, floor-like surface, so
hard that the passing of muLles and wagons scarcely leaves a mark upon it. This hard clay is
principally confined to the central and lower parts of the Desert, but at some places reaches to
the foot of the mountains. It appears to be the extension of the alluvium of the Colorado, and
reaches from its banks, and from the head of the Gulf, as far as the base of San Bernardino
Mountain. Its breadth of surface is variable, but the road from the mouth of the Gila to
Carrizo Creek is upon it for nearly the entire distance. Extensive portions of the Desert are,
however, very different. We find in some places long and gentle slopes bordering the mountains, nearly as in the Great Basin; and in others, level plains, nearly or quite flat, and raised
above the general level of the alluvium or clay. To the traveller, the surface appears nearly
level, but here and there, gentle, local undulations are found, and are caused by accumulations
of blown sand mingled with a portion of clay and partly hardened, so as not to be shifted by the
wind. Other and more recent accumulations form long belts of hills with rounded outlines,
and consist entirely of clean, dry sand, which flows from the hand like water, and is at the
mercy of the wind. The surface of the slopes is variable, but, in general, is gravelly and
firmly impacted, so that it is easily travelled over with wagons. A portion of the slope on the
western side of the Desert, between the emigrant road and the Cohuilla villages, is strewn with
masses of rock, from four to twelve and fifteen inches in diameter; but this was the only point
at which transported fragments were found of such great size. The central or lower portions
of the Desert are entirely free from them. The upper plain, or that part of the Desert north of
Pilot Knob, and at an elevation of about thirty feet above the clay formation, has not the
character of a slope, but is a true plain, with a uniform surface of pebbles, and but little or no
fine gravel or sand. From Fort Yuma this plain is seen on all sides, extending back to the
base of the mountains, and is a perfectly barren waste traversed by the Colorado and the Gila;
their channels and borders of fertile land being marked by green vegetation. South of the
mouth of the Gila, the plain approaches near to the Colorado on its eastern bank, and forms a
long bluff or terrace, similar to that north of the stream, at its bend between the Gila and Pilot
Knob. Thisplainmaybe consideredas the trueor mostperfectdesert,being withoutsoil
vegetation, or water, and the source from which the greater part of the sand is derived.
1 W. H. Emory, U. S. Top. Eng., Rep. Ex. Doe. 30th Cong., 1st sess., p. 98.
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