144 EPPING FOREST
The London Clay forms probably about four-
fifths of the surface of Epping Forest, the remaining
fifth consisting of small patches of sand, boulder
clay, and gravel which lie here and there upon the
London Clay. They are of various geological
dates, and may be seen at various heights above
sea-level. Though they are all comparatively thin,
and geologically obvious mainly from a more
flattened contour of the ground than results from
uncovered London Clay, they give rise to a con-
siderable diversity in the aspect of the Forest from
the space they afford for those trees and plants
which flourish on a sandy, not on a clayey soil.
The London Clay when seen in a fresh cutting
is of a slaty-blue colour, but soon becomes brown
on exposure to the air. In a large section the
layers of concretionary clayey limestone, with septa
of crystalline carbonate of lime, known as Septaria,
are usually to be observed. The greatest thick-
ness attained by the London Clay is in Southern
Essex, and is about 500 feet. At Dagnams, near
Romford, it was found to be 400 feet thick, and
it is probably equally thick at High Beach. It
appears to have been deposited in a comparatively
shallow sea near the mouth of a large estuary,
many of its fossils being vegetable remains from
the nearest land. But crustacea and mollusca
also abound, the most common molluscs belonging
to the great group Gasteropoda, of which the whelk
and snail are familiar examples. Though there
are plenty of fossils in the London Clay, they are
not likely to be easily detected in the sections of
Epping Forest, though conspicuous on the fore-
shore beneath a cliff of London Clay, as at