14 THE NON-MARINE MOLLUSCA OF THE RIVER LEA
Thames, which is generally considered to represent a late
stage of the Pleistocene. The section here given (Fig. I) may
be taken as a typical one. Here the division of the upper bed
into loamy-clay and peat is well shewn.
In our opinion the peat represents the deposit accumulated
in the valley before any reclamation had taken place, whilst the
loamy clay has been deposited by flood-water since man had
altered the original condition by embankments and other works.
Knowing only too well by past experience the difficulties
arising from the shifting of the course of a stream, our work was
principally confined to the parts away fram the influence of the
main stream. It should, however, be borne in mind that the
modern deposits of the River Lea, caused by erosion and
subsequent deposition, differed lithologically from the older beds,
for the former in all cases consisted of sand or sandy gravel with
but little vegetable remains, so that there was never any difficulty
in dealing with such cases. It must also be noted that when a
stream washes away an old deposit, many of the shells, though
apparently fragile, will be drifted down and safely deposited
elsewhere, so that the new bed would contain the shells living
at the time of deposition mixed with those derived from older
beds. That the age of some patches of the shell marl dates
back to Neolithic times is evident from the discovery of
Neolithic flakes in situ under such conditions that they can only
have been dropped where now found from the hand of man. In
18972, we gave to the Essex Field Club an account of the
Mollusca from these beds, yet the new facts accumulated are of
so much importance as to justify the production of this
additional paper.
MOLLUSCA OF THE MODERN DEPOSITS.
The following species were obtained from the loamy-clay:—
Vitrea nitidula (Drap.) Limnaea pereger (Mull.)
Hygromia hispida (Linn.) ,, truncatula (Mull.)
Vallonia pulchella (Mull.) Planorbis marginatus, Drap.
Helicigona arbustorum (Linn.) ,, vortex (Linn.)
Helix nemoralis, Linn. ,, spirorbis, Mull.
Cochlicopa lubrica (Mull.) Bithynia tentaculata, Linn.
Pupa muscorum (Linn.) „ leachii (Shepp.)
Succinea elegans, Risso.
2 Kennard A. S., and Woodward, B. B. The Post-Pilocene Non-marine Mollusca of
Essex. Essex Nat . vol. x., pp. 87—109.