The materials which compose the surface veneer of these plains are coarse conglomerates, gravels, and sands, and the so-called “mortar beds”, which are nothing but sands cemented into sandstone by carbonate of lime. The pebbles in all these deposits are far-traveled and appear to have been derived from erosion of those crystalline rocks which compose the eastern front of the Rocky Mountains. These different materials are not arranged in strictly parallel beds, as are the deposits of a lake or sea; but the beds are made up of long threads of lenticular cross section which are interlaced in the most intricate fashion and which extend down the slope, or outward from the mountain front (Fig. 235). It is thus shown that the High Plains are a bench or plain of alluviation formed at the front of the Rocky Mountains during an earlier series of pluvial periods, and that subsequent uplift has produced the modern river valleys which are cut out of the plain. The plexus of long threads of the coarser materials are the courses of dwindling rivers which interlaced over the former plain, and which in time were buried under other channel deposits of the same nature but in different positions (Fig. 236). The pluvial periods in which this bench was formed produced in other latitudes the great continental glaciers which wrought such marvelous changes in northern North America and in northern Europe. [Illustration: FIG. 235.—Section across the great lenticular threads of alluvial deposits which compose the veneer of the High Plains (after W. D. Johnson).] [Illustration: FIG. 236.—Distributaries of the foothills superimposed upon an earlier series (after W. D. Johnson).] =Character profiles.=—In contrast with the profiles in the landscapes of humid regions (see Fig. 187, p. 177), those of arid lands are marked by straighter elements (Fig. 237). Almost the only exception of importance is furnished by the domes of massive granite monoliths, which are sometimes broken in half by great displacements. Below the horizon the secondary lines in the landscape betray the same straightness of the component elements by the gabled slopes of talus which are many times repeated so as almost to reproduce the lines in a house of cards, since the sloping lines are maintained at the angle of repose of the materials (Fig. 482, p. 443). Wherever the waves of desert lakes have made an attack upon the rocks and have retired the projecting spurs, other gables characterized by slightly different slopes are introduced into the landscape. [Illustration: FIG. 237.—Character profiles in the landscapes of arid lands.] ┌─────────────────────────────────┐ │ PLATE 9. │ │ │ │ [Illustration: THE HIGH PLAINS] │ └─────────────────────────────────┘ READING REFERENCES FOR CHAPTERS XV AND XVI General:—