214 EARTH FEATURES AND THEIR MEANING 



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 ma- 

 terials. 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 model- 

 ing 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 5B and Fig. 226). 

 Such a rain-cut surface is a veritable maze 

 of alternating gully and ridge, a country 

 worthless for agricultural purposes and offer- 

 ing the greatest difficulty in the way of pene- 

 Crating 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). 



Behind the mountain front the valleys out 

 of which the torrents are discharged are usu- 



FIG. 226. -A group of ^ SllOrt ^^ Ste6P side Walls and a rela " 



"demoiselles" in the tively flat bottom, ending head ward in an 

 "bad lands" (after a amphitheater with precipitous walls (Fig. 



photograph by Fair- 227)> j n the wegtem United g^^ such 

 DanKs; . . 



valleys are referred to as " box canons," but 

 in Mohammedan countries the name "wed" applies to the river 

 valley within the mountains and to the distributaries as well. 



Characteristic features of the arid lands. It is characteristic 

 of erosion and deposition within humid regions that all outlines 



