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SURFACE OF THE COLORADO DESERT. 229 
prevailing trend of the granitic elevations to which the region owes its configuration. A pre- 
dominant northwest and southeast trend of the prineipal lines of elevation is said to characterize 
the mountain ranges along the Gila River.! 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 mules 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 moun- 
tains, 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 ehannels 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 itseastern bank, and forms a 
long bluff or terrace, similar to that north of the stream, at its bend between the Gila and Pilot 
Knob. This plain may be considered as the true or most perfect desert, being without soil, 
Vegetation, or water, and the source from which the greater part of the sand is derived. 
1 W. H. Emory, U. 8. Top. Eng., Rep. Ex. Doc. 30th Cong., Ist sess., p. 98. 
