200 THE GEOLOGY OF THE LEA VALLEY.
of gravel which has been brought down in the channel of the stream
and deposited where the current was sluggish. Now and then,
during floods, inundation mud would settle down here and there
above the gravel, and remain as patches of brick-earth. And at a
later date the river, flowing at a slightly lower level, and being more
sluggish than at an earlier period, has ceased to bring down gravel
in its channel, but has deposited instead the alluvium of the marshes
through which it now meanders. We may now consider the evidence
as to the lateral shiftings of the Lea.
In the case of the Thames a glance at the maps of the Geological
Survey shows that from Windsor downwards it now takes on the
whole a more southerly course than it once did. This is made
manifest by the very much greater breadth of river deposits north
than south of the river both above and below London. As regards
the Lea, we find that from its junction with the Stort downwards it
once flowed one, two or even three miles west of its present channel,
while it never ran, on the whole, in its earliest days much further
eastward than it now does. An interesting practical result of this
is that we find a series of old populous villages from Tottenham to
Hoddesdon, on the old river-gravel west of the Lea, along the
course taken by John Gilpin in his involuntary pilgrimage to Ware.
But on the Essex side of the Lea opposite there is no large ancient
village except around Waltham Abbey, where there is a patch of
old river-gravel of exceptional size. For the London Clay, which
usually bounds the marshes on the Essex side, is incapable of
affording any water supply, while the old river gravel at Waltham
Abbey, Cheshunt, Edmonton, and elsewhere, allows water to per-
colate through it, and the underlying London Clay prevents it
from sinking below a moderate depth, at which it can be utilised
by means of a pump. In the days before a knowledge of geology
caused deep borings to be made and companies to be formed for the
supply of water thus obtained, the power of procuring water from
surface gravel was a chief influence in the determination of sites for
villages and even for towns.
Above the junction of the Lea and Stort, the Lea and its
tributaries flow in valleys the sides of which are usually capped
by Boulder Clay. South of the junction Boulder Clay appears
only here and there in outlying patches on the higher ground. It is
almost perfectly certain, however, that Boulder Clay once spread
over the site of the valley of the Lea many miles below the junction