l62
A SKETCH OF THE GEOLOGY OF EPPING FOREST.
From another well-section at Loughton we learn not only some-
thing about the London Clay but about the thickness of the beds
beneath, which do not appear at the surface in our district, though
they may be seen in North Kent, and also in South Essex between
Purfleet and Grays Thurrock. It is given in Mr. Whitaker's Memoir,
already mentioned, and is at the G.E.R. Station.
The London Clay forms, perhaps, four-fifths of the surface of
Epping Forest. It is of a slaty-blue colour when fresh, but soon
becomes brown on exposure to the air. The most prominent objects
in a London Clay section are usually the masses of concretionary
clayey limestone, with septa of crystalline carbonate of lime, known
as Septaria. Fossils are not easy of detection, though abundant in
many places, and conspicuous on the Foreshore below the London
Clay cliffs at Sheppey and Herne Bay. Lowry's Chart of Tertiary
Fossils is indispensable to those collecting shells or other fossils
from the London Clay. It may be purchased from Stanford,
Charing Cross.
The uppermost beds of the London Clay tend to become sandy
like the Bagshot Beds, and thus form "passage beds" between
the two formations. They may be seen in the higher parts of the
Forest, as in Oakhill Pit, about a mile west of Theydon Bois
Station.2 It is often difficult to distinguish between the highest
beds of the London Clay and the lowest of the overlying Bagshot
Series.
2 See description by Mr. N. F. Robarts, "Trans. Essex F. Club," vol. iii., p. 234.