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]).
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.
Fig. 229.—Flat-bottomed basin separating dunes—bajir or takyr (after Ellsworth Huntington).
Next in order is (3) the belt of the flying sand, a zone of dune ridges often separated by narrow, flat-bottomed basins ([Fig. 229]) into which the strongest streams bring the finer sands and silt from the mountains. Lastly, there is (4) the central sink or sinks, into which all water not at once absorbed within the zone of alluviation or in the zone of dunes is finally collected. Here are the true lacustrine deposits of clay and separated salts ([Fig. 230] and [Fig. 207], [p. 201]). The lake deposits fill in all the original irregularities of the desert floor, out of which the tops of isolated ranges of mountains now project like islands out of the surface of the sea. The several zones of deposits in their order from the margin to the center of the desert are given schematically in [Fig. 231].
Plate 8.
