period are widespread. Locally overlying the Eocene strata on the summit of the
Epping Forest Ridge is a variable deposit mapped by the Geological Survey as
Pebble Gravel. The age of this deposit is uncertain, but most workers consider it
to belong to the early Pleistocene and to be at least partially of marine origin
(Hey et al 1971). In Epping Forest it is described as having a coarse sandy
matrix containing flint pebbles with some angular flints and chert, and also
small white quartz pebbles. On Deer Shelter Plain and elsewhere the surface
metre or so is a reddish clay with rounded flint pebbles having similarities to the
Pebbly Clay Drift described by Thomasson (1961) in Hertfordshire. Possibly the
clayey component is derived from Chalky Boulder Clay. There are many old
shallow gravel diggings in the Pebble Gravel.
At lower elevations sands and gravels of slightly different composition to
the Pebble Gravel and considered to be of glacial origin, hence the terminology
Glacial Gravels, occur as isolated outliers principally on the Ching-Roding
interfluve. An exposure in the patch around Loughton Camp revealed bedded
sand and flint gravel with much igneous, metamorphic and other foreign
material. The age and origin of this and other outliers on Long Hills, in parts of
Lords Bushes, Knighton Wood, Chingford Golf Course and Walthamstow
Forest, at varying elevations, is, however, not clear. Small ponds in these gravel
areas may be the result of digging for gravel.
Overlying the Glacial Gravels in much of western and northern Essex is an
extensive spread of Boulder Clay, a glacial till left by retreating ice sheets that
occupied Essex during the Anglian Stage of the Pleistocene ice age. It is a grey
or brown calcareous clay containing common chalk fragments and flints along
with occasional boulders of flint and limestone. Epping Long Green is on part of
the main till sheet, but further south chalky Boulder Clay occurs only as
outliers. There are two such outliers on the summit of the Forest Ridge, one
being on the northern edge of Epping Thicks to the south-west of Bell Common
where chalky clay apparently rests on Pebble Gravel (Ellison, private com-
munication). Boulder Clay also occurs to the south in the Roding Valley making
it probable that ice overtopped the whole of the ridge.
The southernmost (metropolitan) parts of the Forest are on terrace deposits
of the River Thames. These are sandy gravels, with thin seams of stoneless sand,
giving rise to flat level surfaces. Leyton Flats and the higher parts of Wanstead
Park are on the Boyn Hill terrace between 20 and 30m O.D.; Wanstead Flats to
the south is on the Taplow terrace at about 15 m O.D. These spreads of gravel
are thought to have been deposited along earlier courses of the Thames flowing
at higher levels than does the river today. The terraces are frequently covered
with thin layers of loamy brickearth, in part derived from windblown silt, which
is thickest in the Roding valley between Buckhurst Hill and Woodford.
Clayey alluvium occurs in the floodplain of the Roding and extends into
Wanstead Park in the region of the Lakes and Lincoln Island. Alluvium is the
product of overbank discharge of rivers and streams in times of flood, and also
accumulates as point bars on the downstream side of meanders. Small areas
of point bar alluvium occur in the minor stream valleys of the Forest, but these
are of very limited extent and do not extend to developed floodplains.
8