[Illustration: FIG. 223.—Ideal section across the rising mountain wall surrounding a desert and a part of the neighboring slope (after R. W. Pumpelly).]
As the mountains upon seacoasts are generally rising with reference to the neighboring sea bottom, so the mountains which hem in the deserts are generally growing upward with reference to the inclosed desert floor. The marginal dislocations which separate the two are often in evidence at the foot of the steep slope (Fig. 223), and these may even appear as visible earthquake faults to indicate that the uplift is more accelerated than the deposition along the mountain front.
FIG. 224.—Dry delta or alluvial fan at the foot of a mountain range upon the borders of a desert.]
=The zone of the dwindling river.=—The rapid uplift so generally characteristic of desert margins gives to the torrential streams which develop after each cloudburst such an unusual velocity that when they emerge from the mountain valleys on to the desert floor, the current is suddenly checked and the burden of sediment in large part deposited at the mouth of the valley so as to form a coarse delta deposit which is described as a _dry delta_ (Fig. 224). Dependent upon its steepness of slope, this delta is variously referred to as an _alluvial fan_ or _apron_, or as an _alluvial cone_. Over the conical slopes of the delta surface the stream is broken up into numerous distributaries which divide and subdivide as do the roots of a tree. In the Mohammedan countries described as _wadi_, these distributaries upon dry deltas are on the Pacific coast of the United States referred to as “washes” (Fig. 225).
[Illustration: FIG. 225.—Map of the distributaries of neighboring streams which emerge at the western base of the Sierra Nevadas in California (after W. D. Johnson).]
Fast losing their velocity after emerging from the mountains, the various distributaries drop first of all the heavy bowlders, then the large pebbles and the sand, so that only the finer sand and the silt are carried to the margin of the delta. As they enlarge their boundaries, the neighboring deltas eventually coalesce and so form an _alluvial bench_ or “gravel piedmont” at the foot of the range. Only the larger streams are able to entirely cross this bench of parched deposits with its coarsely porous structure, for the water is soon sucked up by the thirsty materials. Encountering in its descent more clayey layers, this water is conducted to the surface near the margin of the bench and may there appear as a line of springs. At this level there develops, therefore, a zone of vegetation, though there is no local rain.