A SKETCH OF THE GEOLOGY OF EPPING FOREST. 161
aries : On the south, the high road between Stratford and Great
Ilford. On the east, the alluvial flat of the Roding as far north as
Chigwell Lane Railway Station, and thence (northward) the Epping
and Ongar Railway as far as Coopersale Common. A line drawn
from Coopersale Common in a north-westerly direction to Roydon
on the Stort may be considered the northern limit, while the western
is formed by the alluvial flats of the Stort and Lea from Roydon to
Stratford.
Within the above boundaries the oldest formation occupying the
surface is the London Clay, though the Woolwich and Reading Beds,
which underlie it, have been met with here and there beneath the
old river gravel which covers so. much ground towards the southern
margin of the Forest. At Dagnams, about three miles N.E. of
Romford, the London Clay was found to be 400 ft. thick, and it
probably reaches an equal thickness in Epping Forest. A well-
section at Loughton, the details of which are given in Mr. Whitaker's
Memoir on "The Geology of London and of Part of the Thames
Valley," vol. ii., p. 25, shows that the London Clay was there
250 feet thick, though its full thickness could not have been ascer-
tained. A boring through the little outlying patch of Bagshot Sands
at High Beach would be the only one within our boundaries giving
the full thickness of the London Clay. For the Lower Bagshot
Beds, which overlie the London Clay at that spot, belong to the
same Eocene series, and rest conformably on the last-named forma-
tion, while the various patches of Gravel and Boulder Clay which
are found elsewhere on the surface, resting upon the London Clay,
are of post-Pliocene age, and consequently highly unconformable.
In other words, the London Clay and the overlying Bagshot Beds
were deposited continuously, there having been no waste of the
upper strata of London Clay previous to the deposition of the
Bagshot Sands. On the other hand, the various post-Pliocene Beds
were laid down only after the London Clay had been subjected to
much erosion during the long interval between the Eocene and post-
Pliocene eras. Consequently only when we bore through Bagshot
Beds, and then through the London Clay beneath, do we ascertain
the full thickness of the latter. (See Fig. 1 Section through High
Beach.) Of course, in sinking wells, people naturally go through as
little London Clay as practicable, and the perusal of well-sections
tends to erroneous notions as to its full thickness unless the foregoing
considerations are remembered.