176 THE ESSEX NATURALIST.
The Ponders End Stage (— 20 to + 50 ?) [Warren, 1912,
1916].—The evidence of the tundra deposits that in part underlie
the alluvium of the Lea Valley is particularly satisfactory. The
greater bulk of the plant remains from these extensive deposits
consists of Arctic Willows, but the flora includes survivals from
more temperate conditions. Other remains include Reindeer,
Siberian Mammoth, Woolly Rhinoceros, Lemming and Alces,
together with shells and mosses that are in harmony with the
tundra.
These deposits are an accumulation of marsh silt, and they
are underlaid and overlaid by tumultuous gravels ; they are
partially broken up by ice-raft action, but large patches remain
undisturbed and it is a mistake to think that the "rafts" were
transported from any distance.
It is noteworthy that the gradient of this Pleistocene marsh,
from Broxbourne through Waltham Cross to Stratford, is
precisely the same as that of the present river, but lies 16 to 18
feet lower.
After many years of search I have secured cumulative evidence
of the Late Levallois industry on the underlying floor below the
Ponders End gravels, but still no industry in the plant beds.
The Buried Channel.—This is a former deep and straight
channel of the Thames, that was completely silted up, to present
high tide level, before the end of the Pleistocene Period, that is.
during the dominance of the Mammoth fauna. It is frequently
encountered in borings from Brentford down-stream to the sea.
The most surprising record of the depth of its floor is 60 feet
below O.D. at Nine Elms, which, if correct, would indicate a
gradient of about 7 feet a mile for the upper reach. In the
Thames Estuary it has not, so far as I am aware, been actually
proved to more than 10 or 20 feet deeper. It is in the normal
course of things for the gradient to flatten out down-stream. In
the Lea Valley the floor of the channel has been proved to 40 ft.
below O.D. at Walthamstow ; that is to say, 40 or 50 feet below
the buried tundra-marsh. I think it probable that the Buried
Channel is trenched through the Ponders End deposits, but this
still awaits proof or disproof.
The Hallsford Loess.—Athough this is only a small local
deposit it is one of exceptional interest which Dr. Zeuner has
analysed ; he finds it to be a true wind-borne loess (a steppe
deposit) of continental type, which was formerly thought to be
absent from this country. The section, which is shown in Fig. 1,
has been described under "Hallsford" (ante).
Immediately under Bed 2 there is a stony line (3 in the section)
representing a former land surface, on which I have found one