72
THE ESSEX NATURALIST.
It has been estimated that London Clay forms four-fifths
of the whole surface of Epping Forest, the remaining fifth
consisting of patches of sand and gravel that cap the clay.
Within the limits of the plateau the ratio between similar surface
layers is as 3:2.
Sand and gravel surfaces throughout the Forest are remark-
ably well indicated by the presence of birch grove, birch oak-
wood, and of small colonies of this tree. These very distinctive
features did not exist 50 years ago.
On entering the Forest from Loughton, by way of Earl's
Path, a few birches stand as sentinels around the gravel pit,
now a pond. Beyond this point they increase rapidly in number
on both sides of the road, but more especially on the western
side; they disappear again by the time the Epping road is
reached. The appearance and disappearance coincide with a
patch of gravel overlying the London Clay. This gravel occurs
just above the 200 feet contour line. It has been described by
our Secretary1 as forming part of a high level terrace of the
Roding.
The birch grove around the church at High Beach is upon
pebble gravel (362 feet) that has been broken up by the com-
paratively recent digging of numerous small, shallow, now
overgrown pits, while the attractive belt of birch that sweeps
round the church and reservoir on the western side, at a slightly
lower level, is for the most part on Bagshot beds. Important
birch groves occur around the Wake Arms and in close proxi-
mity to it, notably one at Jack's Hill. Here again are numerous
overgrown gravel pits with a depth of 3 to 4 feet. The honey-
combed nature of this pit area arises from the fact that the
gravel is deposited in pockets. During a visit to this locality
in October last, I was able to observe a small excavation that
was being made by workmen on the side of the Epping road.
Measurements made at the time gave the following results:—
Surface soil 9", gravel 30", and a depth of clay removed 4".
Birch groves occur also at Genesis Slade, Epping Thicks,
Oak Hill, Piercing Hill, Sand Pit Plain, and on the western
side of Loughton Camp. These are not confined to the Pebble
Gravel, but the soil is at least sandy.
The gravels at Jack's Hill and its vicinity have a wide
3 Thompson, Percy G., "On the Occurrence of Rhaxella Chert in Epping Forest Gravels."
Essex Naturalist, 1913, pt. x.-xii., vol. xvii. p. 256.