=Deflation.= In the denudation of an arid region, wind erosion is comparatively ineffective as compared with deflation (Latin, _de_, from; _flare_, to blow),--a term by which is meant the constant removal of waste by the wind, leaving the rocks bare to the continuous attack of the weather. In moist climates denudation is continually impeded by the mantle of waste and its cover of vegetation, and the land surface can be lowered no faster than the waste is removed by running water. Deep residual soils come to protect all regions of moderate slope, concealing from view the rock structure, and the various forms of the land are due more to the agencies of erosion and transportation than to differences in the resistance of the underlying rocks. [Illustration: Fig. 129. Mesa Verde, Colorado In the distance on the left are high volcanic mountains. On the extreme right are seen outliers of strata which once covered the region of the mesa] But in arid regions the mantle is rapidly removed, even from well-nigh level plains and plateaus, by the sweep of the wind and the wash of occasional rains. The geological structure of these regions of naked rock can be read as far as the eye can see, and it is to this structure that the forms of the land are there largely due. In a land mass of horizontal strata, for example, any softer surface rocks wear down to some underlying, resistant stratum, and this for a while forms the surface of a level plateau (Fig. 129). The edges of the capping layer, together with those of any softer layers beneath it, wear back in steep cliffs, dissected by the valleys of wet-weather streams and often swept bare to the base by the wind. As they are little protected by talus, which commonly is removed about as fast as formed, these escarpments and the walls of the valleys retreat indefinitely, exposing some hard stratum beneath which forms the floor of a widening terrace. The high plateaus of northern Arizona and southern Utah (Fig. 130), north of the Grand Canyon of the Colorado River, are composed of stratified rocks more than ten thousand feet thick and of very gentle inclination northward. From the broad plat form in which the canyon has been cut rises a series of gigantic stairs, which are often more than one thousand feet high and a score or more of miles in breadth. The retreating escarpments, the cliffs of the mesas and buttes which they have left behind as outliers, and the walls of the ravines are carved into noble architectural forms--into cathedrals, pyramids, amphitheaters, towers, arches, and colonnades--by the processes of weathering aided by deflation. It is thus by the help of the action of the wind that great plateaus in arid regions are dissected and at last are smoothed away to waterless plains, either composed of naked rock, or strewed with residual gravels, or covered with drifting residual sand.