[Illustration: FIG. 233.—Exhumed structures in the buried city of Nippur (after Hilprecht).]
=The origin of the high plains which front the Rocky Mountains.=—To the eastward of the great backbone of the North American continent stretches a vast plain gently inclined away from the range and generally known as the High Plains region (plate 9). The tourist who travels westward by train ascends this slope so gradually that when he has reached the mountain front it is difficult to realize that he has climbed to an altitude of five thousand feet above the level of the sea. That he has also passed through several climatic zones—a humid, a semiarid, and an arid—and has now entered a semiarid district, is more easily appreciated from study of the vegetation (Fig. 234). The surface of the High Plains, where not cut into by rivers, is remarkably even, so that it might be compared to the quiet surface of a great lake.
[Illustration: FIG. 234.—Section across the High Plains, showing the position of the terrace and the climatic zones above it (after W. D. Johnson).]
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.