The Ancient Fauna of Essex.
9
occupies. Every stream is converted by the Beavers into a
series of falls with beaver-dams, and large, deep, clear, still
pools of water. It is evident that, if ever a big flood
came down such rivers, these beaver-dams would be quite
calculated to cause an overflow and an inundation of the
Forest for miles around. Many of the forest-trees could not
stand this excessive damp, and would give way to Sphagnum
or bog-moss ; and large tracts thus flooded would be converted
into peaty, marshy fen-land. That I believe to be the origin
of the fen-lands over a great part of our Eastern counties. Of
course, geologically, there is another aspect of the question
which we must not omit to mention ; that is, the strata in
this area are of a soft clayey, chalky, or sandy character, and
lend themselves admirably to the action of denuding agents :
but I have no doubt that the Beaver helped these agents
most effectively. By damming back the water of its streams
it destroyed the Forest, and converted it by degrees into a
primitive marsh.7
We must now pass from the consideration of the series of
deposits characterised by the fauna nearest to that existing
at the present day to those belonging to an earlier date, but
which are also found in the valleys of the Thames, the Lea,
and the Roding, and which underlie the more superficial
deposits of which we have already spoken. They consist
mainly of gravels, sands, and brick-earth, and are all of
fluviatile origin, being marked by the presence of land and
7 "The flow of streams is sometimes interfered with, or even diverted by
the operations of animals. Thus the Beaver, by cutting down trees
(sometimes one foot or more in diameter) and constructing dams with
the stems and branches, checks the flow of water-courses, intercepts
floating materials, and sometimes even diverts the water into new
channels. This action is typically displayed in Canada and in the
Rocky Mountain regions of the United States. Thousands of acres in
many valleys have been converted into lakes, which, intercepting the
sediment carried down by the streams, and being likewise invaded by
marshy vegetation, have subsequently become morass, and finally meadow-
land. The extent to which, in these regions, the alluvial formations of
valleys have been modified and extended by the operations of the Beaver
is almost incredible'' (Geikie's 'Geology,' p. 455). See also 'The American
Beaver and his Works,' by Lewis H. Morgan, Philadelphia, 1868,