Every river gives origin to deposits of this kind at
different points along the course of its valley; and it is not
uncommon to find that there exist in the valley of a single river
two or more sets of these gravel-beds, formed by the river itself,
but formed at times when the river ran at different levels, and
therefore formed at different periods. These different accumulations
are known as the "high-level" and "low-level" gravels; and a
reference to the accompanying diagram will explain the origin
and nature of these deposits (fig. 255). When a river begins
to occupy a particular line of drainage, and to form its own
channel, it will deposit fluviatile sands and gravels along its
sides. As it goes on deepening the bed or valley through which
it flows, it will deposit other fluviatile strata at a lower
level beside its new bed. In this way have arisen the terms
"high-level" and "low-level" gravels. We find, for instance, a
modern river flowing through a valley which it has to a great
extent or entirely formed itself; by the side of its immediate
channel we may find gravels, sand, and loam (fig. 255, 2 2')
deposited by the river flowing in its present bed. These are
_recent_ fluviatile or alluvial deposits. At some distance from
the present bed of the river, and at a higher level, we may find
other sands and gravels, quite like the recent ones in character
and origin, but formed at a time when the stream flowed at a higher
level, and before it had excavated its valley to its present
depth.
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