[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. The alluvial bench grows upward by accretion of layers which are thickest at the mountain end, so that the steepness of the bench increases with time. =Erosion in and about the desert.=—The violent cloudburst that is characteristic of the arid lands is a most potent agent in modeling the surface of the ground wherever the rock materials are not too firmly coherent. Under the dash of the rain a peculiar type of “bad land” topography is developed (plate 5 B and Fig. 226). Such a rain-cut surface is a veritable maze of alternating gully and ridge, a country worthless for agricultural purposes and offering the greatest difficulty in the way of penetrating it. When composed of stiff clay with scattered pebbles and bowlders, the effect of the “rain erosion” is to fashion steep clay pillars each capped by a pebble and described as “demoiselles” (Fig. 226). [Illustration: FIG. 226.—A group of “demoiselles” in the “bad lands” (after a photograph by Fairbanks).] Behind the mountain front the valleys out of which the torrents are discharged are usually short with steep side walls and a relatively flat bottom, ending headward in an amphitheater with precipitous walls (Fig. 227). In the western United States such valleys are referred to as “box cañons”, but in Mohammedan countries the name “wed” applies to the river valley within the mountains and to the distributaries as well.