The high plains.  The rivers which flow eastward from the Rocky Mountains have united their fans in a continuous sheet of waste which stretches forward from the base of the mountains for hundreds of miles and in places is five hundred feet thick (Fig. 80).  That the deposit was made in ancient times on land and not in the sea is proved by the remains which it contains of land animals and plants of species now extinct.  That it was laid by rivers and not by fresh-water lakes is shown by its structure.  Wide stretches of flat-lying, clays and sands are interrupted by long, narrow belts of gravel which mark the channels of the ancient streams.  Gravels, and sands are often cross bedded, and their well worn pebbles may be identified with the rocks of the mountains.  After building this sheet of waste the streams ceased to aggrade and began the work of destruction.  Large uneroded remnants, their surfaces flat as a floor, remain as the High Plains of western Kansas and Nebraska.

River deposits in subsiding troughs.  To a geologist the most important river deposits are those which gather in areas of gradual subsidence; they are often of vast extent and immense thickness, and such deposits of past geological ages have not infrequently been preserved, with all their records of the times in which they were built, by being carried below the level of the sea, to be brought to light by a later uplift.  On the other hand, river deposits which remain above baselevels of erosion are swept away comparatively soon.

The great valley of California is a monotonously level plain of great fertility, four hundred miles in length and fifty miles in average width, built of waste swept down by streams from the mountain ranges which inclose it,—­the Sierra Nevada on the east and the Coast Range on the west.  On the waste slopes at the foot of the bordering hills coarse gravels and even bowlders are left, while over the interior the slow-flowing streams at times of flood spread wide sheets of silt.  Organic deposits are now forming by the decay of vegetation in swampy tule (reed) lands and in shallow lakes which occupy depressions left by the aggrading streams.

Deep borings show that this great trough is filled to a depth of at least two thousand feet below sea level with recent unconsolidated sands and silts containing logs of wood and fresh-water shells.  These are land deposits, and the absence of any marine deposits among them proves that the region has not been invaded by the sea since the accumulation began.  It has therefore been slowly subsiding and its streams, although continually carried below grade, have yet been able to aggrade the surface as rapidly as the region sank, and have maintained it, as at present, slightly above sea level.