164 A SKETCH OF THE GEOLOGY OF EPPING FOREST.
We now come to deposits of Glacial Age. These consist of the
Chalky Boulder Clay, which covers so large a proportion of the sur-
face of the eastern counties from mid-Essex northward, and the sand
and gravel associated with and underlying it. But though this
Boulder Clay covers a considerable area east of the Forest, as far
south as Chigwell and Havering-atte-Bower, and westward, in
Middlesex, as far south as Finchley, it is confined to the north-
eastern border of the Forest district. Scarcely any has been
detected south-west of a straight line drawn from Abridge on the
Roding through Nasing to the Stort. But the geological surveyors
are inclined to identify some gravel patches of considerable size at
Loughton, Buckhurst Hill, and Woodford with the Glacial Gravel
usually found beneath the Boulder Clay. Small deposits of
Boulder Clay have been elsewhere occasionally found in this gravel.
Possibly some day some may be detected in or on this supposed
Glacial Gravel of the Forest district.
As regards composition, the Glacial Gravel consists chiefly of
flint, sub-angular or rounded, quartz and quartzite, flint pre-
dominating. Sometimes rolled fossils from the Lias and Oolite
have been found in it. It varies very much in appearance, being at
one spot well stratified, at another showing few, if any, signs of
stratification. And at one section it may be mainly coarse gravel, at
another chiefly sand.
The Boulder Clay is often called the Chalky Boulder Clay from
the great number of Chalk pebbles it contains, most of which show
glacial scratches. In a deep section this formation can hardly be
mistaken for any other, though within two or three feet of the surface
the Chalk and other limestone rocks have usually been dissolved
away by the action of rain and its most marked characteristic thus
removed. Where well shown it is seen to be a confused mass of
rocks of various ages in a matrix of bluish-grey or buff-coloured
clay. Chalk and flint predominate, then come fragments of rock
and fossils belonging to the Lias and Oolite formations. Belem-
nites and Gryphaeas are very common. It is evident that the rocks
found in the Boulder Clay have been derived from the north, not
from the south. But it is impossible to discuss here the various
theories held by geologists as to the exact kind of ice action that
has produced the Chalky Boulder Clay of the Eastern Counties.
Some consider the chief agent to have been an ice-sheet, others
incline to believe in icebergs or coast-ice.