Every river produces at the present day beds of fine mud and
loam, and accumulations of gravel, which it deposits at various
parts of its course--the gravel generally occupying the lowest
position, and the finer sands and mud coming above. Numerous
deposits of a similar nature are found in most countries in various
localities, and at various heights above the present channels of
our rivers. Many of these fluviatile (Lat. _fluvius_, a river)
deposits consist of fine loam, worked for brick-making, and known
as "Brick-earths;" and they have yielded the remains of numerous
extinct Mammals, of which the Mammoth (_Elephas primigenius_) is
the most abundant. In the valley of the Rhine these fluviatile
loams (known as "Loess") attain a thickness of several hundred
feet, and contain land and fresh-water shells of existing species.
With these occur the remains of Mammals, such as the Mammoth and
Woolly Rhinoceros. Many of these Brick-earths are undoubtedly
Post-Glacial, but others seem to be clearly "inter-glacial;" and
instances have recently been brought forward in which deposits
of Brick-earth containing bones and shells of fresh-water Molluscs
have been found to be overlaid by regular unstratified boulder-clay.
The so-called "Valley-gravels," like the Brick-earths, are fluviatile
deposits, but are of a coarser nature, consisting of sands and
gravels.
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