58 ALLUVIAL AND OTHER RECENT DEPOSITS AT FELSTEAD.
washed them all into the valley where the sluggish stream, being
inadequate to great carrying power, left the heavy material behind.
Incidentally it may be noticed that the river-bed was raised in con-
sequence, and for a few times it may have oscillated quite across the
alluvium.
At this stage we may ask the question, When did the plants and
animals put in their appearance ? When the conditions of climate
and proximity allowed of it, is the stereotyped reply. It seems, how-
ever, that we have left in these alluvial deposits a page or two
of the book that might furnish a better answer. I have found
shells at eight feet deep in undisturbed alluvium. Where they end,
or rather where they begin, in the deposit we do not know. Perhaps
it could be shown that they do or do not begin at the bottom ; at
any rate, it seems worth while to try and determine this point.
There can be little doubt that existing climatal conditions obtained
throughout the period of deposition.
The alluvium along the Chelmer Valley at Felstead is at places a
quarter of a mile wide, and varies from that to a mere strip, or to
none at all. Some gravels occur in it occasionally, but they are
very local, and are either the result of freshets or of brooks flushing
down glacial gravels into the main stream. In nearly all cases their
origin can be readily determined, nor can they be possibly mistaken
for Glacial beds at any time, for they always contain stones coated
with carbonate of lime, which never occur in Glacial beds. There
are also lenticular masses of silt present, and these generally contain
the most fossils. Their origin can be traced by the observation of a
winter's flood; an eddy in the swollen stream, which is just over-
flowing its banks, collects the dead shells in great quantities, and
carries them out of the current into still pools where clay is being
deposited.
The greatest depth of the alluvium is unknown ; perhaps it may
attain twenty feet in places, but sections, except that made by the
river, are rare.
Of the organic remains entombed in the river alluvium, we are in
part ignorant. Bones of mammals are known to occur; shells of the
mollusca are frequently exposed, and with trifling exceptions are
those of species' now occupying the stream. The exceptions, so far
as my personal observation goes, are Limnaea auricularia and Neritina
fluviatilis. The latter is a common fossil, but if either of the species
mentioned now exist in this locality they are very rare. There is also