ON SOME ANCIENT LAKE REMAINS AT FELSTEAD, ETC. 35
cutting of the new channels exposed some sections of soil varying
from two to ten feet in depth, and as these occur on both sides of the
dam, we are enabled to study the deposits of the lake and the con-
temporaneous deposits which formed on the other or outer side of
the dam. I may here mention that at the time the workmen widened
the old breach of the dam they found some curious implements
which they took to be "Mill-bills," but which equally well might
have been "Celts." These relics are lost.
The dam is about 150 yards long, about 35 feet broad at its base,
and 10 feet at its top, and is about 20 feet high at its deepest part.
It is made most probably of Gravel and Boulder-clay in equal layers,
and would hold back perhaps 10 acres of water or more.
Of the first deposit inside the dam we cannot speak with certainty,
as the channel does not cut through it. At one place the brook
flows over very compact dark lake mud, destitute of stones, and nearly
so of organic remains. Traces of oyster-shells may be noticed
sparingly, and there are the nacreous remains of some fresh-water
molluscs. The corresponding level at a point higher up stream is of
gravel which as yet has furnished no relics.
On the outside of the dam the original bottom is exposed in the
section cut by the stream. It is of Chalky Boulder Clay. Paren-
thetically it may be observed that Boulder Clay occupying the
bottom of a valley is rare. The sheet of Boulder Clay also extends up
the northern side of the valley slope to a considerable elevation. It is
necessary to bear this in mind.
Soon after the dam was made, perhaps immediately after, a fire
was lit on this newly scooped out Boulder Clay, and the fragments
of charcoal and calcined stones resulting remain to this day. It
would seem that the place of this fire, occupying a depression, soon
became saturated with water, because in immediate contiguity to
these ashes is a thin layer of shell-marl. The species occurring are
Land-shells of various species; among them occur Helix arbustorum
and Cyclostoma elegans, both of which are now considered to be
locally extinct.
The first overflow of the dam has left unmistakable evidence of
its work. This is a bed of rather coarse gravel of about two feet in
thickness. It would seem that the first overflow came from the
northern end of the dam, and eroded some Glacial gravel at that
point. It then passed down the slope of the valley in a line parallel
and near to the side of the dam and laid clown the gravel in its
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