WITH ESPECIAL REFERENCE TO THOSE OF ESSEX.
29
of the Thames and Lea being by far the most important. The value
of these deposits lies not so much in their agricultural qualities as in
the advantages they offer as the sites of villages or towns compared
with the London Clay, which, in Essex, they overlie. These river
drifts consist of sand, gravel and brickearth—mainly sand and gravel
—deposited by the rivers Thames and Lea when they had more
power to carry down material than they now have, and had not cut
their way downwards to their present level. If we look at the geology
of the valleys of the Thames and Lea (as it appears on the drift
map), we see in addition to the alluvium of the marshes—common
to drift and driftless maps—a somewhat broad band of a reddish-
brown colour, with patches on it here and there of light yellow, the
brown being the gravel and the yellow the brickearth. These are the
strata in which Mr. Worthington Smith and others have found so
many flint implements. Looking along the valley of the Lea, we
see that above Tottenham this river drift is almost wholly on the
western side, the eastern showing little but alluvium and London
Clay. South of Tottenham, river gravel abounds almost equally on
both sides. In connection with this distribution of the river drift,
we may also notice that west of the Lea there is an almost continuous
series of populous villages between London and Hoddesdon. East
of the Lea, Waltham Abbey is the only place of any size north of
Tottenham, and it is situated not on the London Clay, but on one of
the only two considerable patches of gravel to be seen on that bank
of the river. Descending below Tottenham to the junction of the
Thames and Lea, the drift map shows clearly that London stands
where it does because of the existence of a broad spread of old river
gravel there, extending to the bank of the Thames. If we look down
the course of the Thames, we nowhere see, either on the Essex or
the Kentish shore, any spot offering advantages in any way worthy of
comparison with those of London as the site of a city of the first rank.
Below London there is no broad expanse of sufficiently elevated river
gravel close to the stream ; higher up there is plenty of river gravel,
but the Thames speedily becomes narrower and less navigable for
ships of any size. Crossing the Lea into Essex, we find that the
broad marshes between its outfall and Purfleet prevent the broad
band of river gravel from being the site of any important port. But
on this gravel are Leyton, Leytonstone, Wanstead, and Romford
towards its northern edge, and Stratford, Plaistow, and Barking to-
wards its southern boundary. Of course, when water companies and