=Characteristic features of the arid lands.=—It is characteristic of erosion and deposition within humid regions that all outlines become softened into flowing curves, due to the protective mat of vegetation. In arid lands those massive rocks which are without structural planes of separation, partly as a consequence of exfoliation, develop broad domes which are projected upon the horizon as great semicircles, broken in half it may be by displacement. The same massive rocks where intersected by vertical joint planes yield, on the contrary, sharp granite needles like those of Harney Peak (plate 8 A). Similarly, schistose or bedded rocks, when tilted at a high angle, may yield forms which are almost identical. Examples of such needles are found in the Garden of the Gods in Colorado. At lower levels, where the flying sand becomes effective as an eroding agent, flat bedded rocks become etched into shelves and cornices, and if intersected by joints, the shelves and cornices are transformed into groups of castellated towers and pinnacles of a high degree of ornamentation. These fantastic erosion remnants are usually referred to as “chimneys” and may be seen in numbers in the bad lands of Dakota, as they may in Colorado either in Monument Park or in the new Monolithic National Park (plate 8 B). [Illustration: FIG. 227.—Amphitheater at the head of the Wed Beni Sur (after Walther).] Where wind erosion plays a smaller rôle in the sculpture, but where after an uplift a river has made its way, horizontally bedded rocks are apt to be carved into broad _rock terraces_, nowhere shown upon so grand a scale as about the Grand Cañon of the Colorado. Each harder layer has here produced a floor or terrace which ends in a vertical escarpment, and this is separated from the next lower layer of more resistant rock by a slope of talus which largely hides the softer intermediate beds. The great Desert of Sahara is shaped in a series of rock terraces or steppes which descend toward the interior of the basin. A single harder layer of resistant rock comes often to form the flat capping of a plateau, and is then known as a _mesa_, or table mountain. Along its front, detached outliers usually stand like sentinels before the larger mass, and according to their relative proportions, these are referred to either as small mesas or as the smaller _buttes_ (Fig. 228). [Illustration: FIG. 228.—Mesa and outlying butte in the Leucite Hills of Wyoming (after Whitman Cross, U. S. G. S.).] =The war of dune and oasis.=—In every desert the deposits are arranged in consecutive belts or zones which are alternately the work of wind and water. Surrounding the desert and upon the flanks of the mountain wall there is found (1) the deposit of loess derived from the dust that is carried out of the desert by the wind. Immediately within the desert border at the base of the mountains is (2) the zone of the dwindling river with its sloping bench of coarse rubble and gravel.