112 NOTES ON THE GEOLOGY OF MALDON AND
deep wells. Passing to the Eocene formations, we find that the
Thanet and Reading Beds occupy but a small area along their
southern outcrop between Purfleet and Stanford-le-Hope, and that
only traces of them are visible here and there along a line between
Bishop's Stortford and Sudbury, where their northern outcrop is much
hidden by Glacial Drift. Close to their line of outcrop, both north
and south, is that of the London Clay. But the Thanet and Reading
Beds form but a very small portion of the surface of the county,
while the London Clay not only exists at a moderate depth under a
large portion of northern Essex, but also occupies the greater part
of the surface in the southern and eastern districts. Above the
London Clay, the Bagshot Beds exist but as small outlying patches
south of a line drawn from Chipping Ongar to Chelmsford and
Maldon; while the Pliocene Crag occupies a still smaller portion of
the surface in north-east Essex.
We now come to the Glacial and other Drifts. The loam, gravel,
sand, and Boulder-clay belonging collectively to the Glacial Drift,
occupy more than half the surface of Essex. The other superficial
beds consist of various out-lying, high level gravel patches of no great
area; of the old river-gravel, which forms extensive flats between
Barking and Romford, and elsewhere, and the alluvium of the flats at
Canvey Island, Foulness, and Dengie, and of the various river valleys.
Examination thus discloses that the surface rocks of Essex consist
mainly of London Clay and Glacial Drift, the former covering a large
area in southern and eastern Essex, and being itself covered by
deposits of Glacial Drift age in the north and west. In the latter
region the Drift forms an almost continuous sheet, the London Clay
being visible only here and there in the bottoms of the river valleys.
South of a line drawn from Epping to Maldon, denudation has
exposed the London Clay in the valleys, and confined the Glacial
Drift mainly to the higher ground. And south of Havering, Brent-
wood, and Billericay, the Drift entirely disappears.
Maldon stands just on the edge of the north-westerly or Glacial
Drift district. Its position topographically and geologically is such
that it seems to me certain that its unique advantages as the site of a
port on the Blackwater Estuary must have been recognized from the
very earliest times. Indeed, most of these advantages must have
been much more highly esteemed ten or twenty centuries ago than
they now are, and must have shown the more conspicuously on
account of their absence elsewhere along the course of the estuary.