30
NOTES ON DRIFT MAPS,
the sinking of deep Artesian wells were rarities, the inhabitants of
a district were dependent for their water supply either on streams,
the conservation of rain-water, or the water-bearing qualities of the
strata on which their houses stood. It necessarily happened then
that no sites were more popular, and, consequently, populous, than
those on old river gravel, near a stream of importance, the gravel
being sufficiently elevated to be beyond the reach of floods, and
usually affording a water supply obtainable by means of a pump.
But the driftless map, which ignores every superficial deposit but
marsh alluvium, is singularly misleading with regard to the reason
for the existence of the various centres of population on the Thames
and Lea. For any one trusting to it for information on this point
would learn that both London and the Essex towns and villages
mentioned stood upon the London Clay.
We thus learn that, whatever may be the merits and advantages of
driftless maps in the mountain or mining districts, in Essex they
are not only extremely imperfect but highly misleading, the super-
ficial being the practically important formations. The driftless maps,
as we have seen, recognise the existence but of the following rocks :
Chalk, the Eocene Beds, including the Thanet Sand, Woolwich and
Reading Beds, London Clay, and Bagshot Series, and the Pliocene,
or the Coralline and Red Crags. The one superficial bed shown is,
as before remarked, the alluvium of the marshes, whether bordering
either the rivers or the sea. The drift maps, on the other hand,
show, in addition, the sand, gravel, loam, and Boulder Clay of the
Glacial Drift, sundry patches of gravels of various ages, the oldest
being possibly pre-glacial, and the newest certainly post-glacial.
Lastly, we have the gravel and brickearth of the Thames and Lea.
Of all these deposits the most important are the Glacial Drift and
the old river gravel. Some of the maps of north-eastern Essex have
never been published, except as Drift Maps. But sheets Nos. 1
and 47 have been issued both with and without the drift, and they
include fully two-thirds of this county. The Geological Survey
authorities have kindly lent me the driftless copies of those sheets,
in order that the differences between the drift and driftless editions
may be made the more manifest. On the driftless map we see
scarcely anything but Chalk and London Clay, its dull aspect being
only slightly relieved by a few small patches of Bagshot Beds about
Brentwood and elsewhere, by the alluvium of the Thames and Lea,
and by a thin band of brown, darker in tint than that of the London