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WALLACE H. FULLER
tion. They are found extensively in Chad, Mauritania, Mali, Niger, and
Sudan (Dieleman and deRidder, 1964) as deep, clayey material, weakly
developed on level topography. Where the clay is unusually fine and permeability is impaired, saline soils develop.
Alluvial deposits that form river valleys account for a large total area
of desert soils (Denny, 1967). Sand, silt, and clay conspicuously stratify
in horizontal directions as layers of graded particle sizes. Thin layers of
clay, for example, will overlie silt which is over sand and finally gravel
or coarse sand, just as one might find in miniature in a mud puddle in
a rutty road. Particulate separations may occur in valley soils depending
on the intensity of the water flows that deposited the material (see Fig.
5). Such stratifications lower water infiltration rate and often accumulate
salts in the fine-textured layers.
B. SAND
1. Characteristics
To most people, the desert is sand and sun, and indeed these are most
characteristic of some desert areas such as that depicted in Fig. 6. Although sand is a common property of deserts, the sand of wasteland, of
shifting, moving "vagabond" dunes as seen in movies, is not as common
in all deserts as in some, such as the Sahara. Other surfaces, just as spectacular, and productive soils with profile development inhabit large desert
areas. Moving sands are not classified as soils unless they support vegetaFig. 5. Eroded alluvial plains showing stratified materials.