THE GEOLOGY OF THE LEA VALLEY.
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with the Stort, as outliers of considerable size may be seen as far
south as Chigwell on the east and Finchley on the west. It can
scarcely be doubted, therefore, that the valley of the Lea with the
series of river-deposits which it contains are post-Glacial in age, in
the sense that they are of later date than the Boulder Clay of Essex
and Middlesex. Perhaps I may be allowed to add that in The
Essex Naturalist (vol. vii., pp. i—13) I have given some account of
the evidence pointing to a similar conclusion in the case of the
Thames. Of course it is quite possible that there may have been
more or less low ground coinciding perhaps, here and there, with
that of the present valleys of the Thames and Lea, in pre-Glacial
times. But the evidence decidedly favours the view that the valleys
as we now see them, with the series of river deposits at various
levels which they contain, are post-Glacial in the only sense in which
that term can be locally applied.
Just below Tottenham High Cross the gravel flats of the Lea
become continuous with those of the Thames ; it therefore becomes
impossible to say whether the gravel about Walthamstow should be
considered to belong to the Thames rather than to the Lea, or the
reverse. From this point as far as the outfall of the Lea into the
Thames we may note, west of the marshes of the Lea, the broad
expanse of old river gravel on which London stands. And a
glance at the map of the Geological Survey will show us that
geological conditions which, as we have seen, led to the settlement
of a large village population on the western flank of the valley of the
Lea, equally decided the position of the great city of the Thames.
London has now far outgrown the limits of the site which must have
attracted inhabitants from the very earliest times. But if we confine
our attention to ancient London, now known as the "City," we learn
from the geological map that, as we ascend the Thames, this is the
first spot we come to which consists of a broad spread of river-gravel
of good elevation and yet close to the river ; at once capable of
being made into a place of strength and of yielding a good supply
of water. And we must not forget that ancient London, besides
the advantages already mentioned, had another in the existence of
the river Lea on its eastern side. For its presence furnished the
Londoners both with a valuable defensive outwork and with an
additional watery highway.