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Wind erosion becomes geologically important only in certain localities, and the conditions favoring it are a dry climate and a topography of abrupt and broken reliefs.
On plains where the ground is dry and vegetation scanty or absent, ordinary strong winds are apt to slowly wear into the soil, where the roots of plants do not protect is. If such soil contains sand which is too coarse to be lifted up and carried away, dunes are formed, and the uneven topography thus developed still more favors wind erosion; for it is evident that the slopes of the dunes will be struck with greater force than the even surface of a level plain. In such places the sand grains are triturated and worn, and the abraded material is promptly removed. It is also evident that where a country is traversed by vertical escarpments and cliffs, and steep slopes, strong eddies are set up as the wind strikes these reliefs. Where the rocks are of fine materials and but little indurated, like most of the Mesozoic and Cenozoic beds of the west, it would be singular if such eddies did not erode the bare surfaces of their outcrops. It does not appear practicable to estimate separately the erosion produced by impact of the air alone, and the abrasion produced by the materials carried. The ratio between the two will, of course, vary with the quantity of the load. Where this is considerable, abrasion is no doubt proportionally greater than in water, for the speed of the impinging particles is here much higher, and their striking force consequently greater. Occasionally this circumstance greatly intensifies aerial erosion and produces a natural sand-blast, which is very effective in its action on solid rock. That such abrasion becomes appreciable and important along the escarpments of "mesas" in dry regions appears not to admit of a doubt. In such places the driven sand may sometimes be felt smiting the exposed skin of the traveler.
The speed of the wind being lowest near the surface of the ground, materials must by some means by lifted through this zone of low velocity in order to be transported any considerable distance by the atmosphere.
According to some observations made by Stevenson, the aver-