These deposits consist of broad, flat-lying sheets of clay and
fine sand left by the overflow at time of flood, and traversed
here and there by long, narrow strips of coarse, cross-bedded
sands and gravels thrown down by the swifter currents of the
shifting channels. Occasional beds of muck mark the sites of
shallow lakelets or fresh-water swamps. The various strata also
contain some remains of the countless myriads of animals and
plants which live upon the surface of the plain as it is in
process of building. River shells such as the mussel, land shells
such as those of snails, the bones of fishes and of such land
animals as suffer drowning at times of flood or are mired in
swampy places, logs of wood, and the stems and leaves of plants
are examples of the variety of the remains of land and fresh-water
organisms which are entombed in river deposits and sealed away as
a record of the life of the time, and as proof that the deposits
were laid by streams and not beneath the sea.

BASIN DEPOSITS

DEPOSITS IN DRY BASINS. On desert areas without outlet to the sea,
as on the Great Basin of the United States and the deserts of
central Asia, stream-swept waste accumulates indefinitely. The
rivers of the surrounding mountains, fed by the rains and melting
snows of these comparatively moist elevations, dry and soak away
as they come down upon the arid plains. They are compelled to lay
aside their entire load of waste eroded from the mountain valleys,
in fans which grow to enormous size, reaching in some instances
thousands of feet in thickness.

The monotonous levels of Turkestan include vast alluvial tracts