4 6 PHYSICAL GEOLOGY , 



of wind and a new surface is exposed which is, in turn, planed by the 

 blown sand. The pebble may be turned over by the undermining 

 of the sand on one side, when the pebble falls into the depression thus 

 made, and the wind is permitted to plane another surface. Pebbles of 

 this sort are sometimes found in ancient rocks and afford evidence of 

 the physical conditions of the time in which they were deposited. 



Wind scour wears away the softer strata of a desert much more 

 rapidly than the harder, producing wide plains above which the 

 harder rocks stand as isolated hills. In areas composed of horizontal 

 strata the soft rocks are removed, and the region is lowered to a harder 

 stratum, which may, in turn, be cut up and the whole region reduced 

 to a still lower level. During this gradual lowering, the deserts are 



Fig. 24. — Diagram showing a table such as that appearing in Figure 23, formed 

 by the wearing away of the beds A and B, by wind-blown sand. 



eroded, leaving extensive, flat-topped elevations capped by harder 

 rock (Figs. 23, 24), which are later cut up into conical hills and finally 

 destroyed. 



In desert regions where plains predominate, we sometimes find 

 mountains whose bases are covered for 1000 to 2000 feet with sand, 

 rising above the plains. (McMahon.) In places we find also bare 

 rock exposed in the basins, showing that the wind is able to exca- 

 vate the rock to low levels. In fact, it seems probable that so long 

 as the ocean is held back from a desert, eolian excavation may go 

 below sea level. The limit to eolian excavation is the level of under- 

 ground water (p. 56), for when that is encountered, wind erosion is 

 ineffective, since the sand is then held by the moisture and further 

 removal prevented. 



Sand Dunes. — When wind meets an obstacle, its velocity is 

 lessened and, if it carries sand, some of its burden is dropped. The 

 mounds of sand which are thus piled up by the wind are called dunes. 

 Sand dunes are most abundant (1) in deserts, being as a rule more 

 numerous in low-lying areas ; (2) on sandy coasts where the prevailing 

 winds are on shore; and (3) near the beds of rivers whose volume 

 varies, leaving broad areas of sand exposed during the dry season. 

 Dunes of this origin are common in Nebraska, Kansas, Mexico, and 

 many other regions. Sand dunes occur in the above-mentioned 



