260
THE ESSEX NATURALIST.
ice surface, either on its margin or within the main mass,
it is implied that close search has shewn such debris to be
absent. It is, for instance, absent from the summits of nearly
all the Bagshot outliers centering around Brentwood. In the
light of such evidence it is possible to plot the ice margin with
confidence for many miles. The results are feasible and con-
sistent, and independent confirmatory evidence bearing upon
the direction of ice-motion is forthcoming in several instances.
Some uncertainty must necessarily exist as to the limit of certain
ice tongues pushing out into low ground, but in many cases the
terminal gravel-fan is present in force and puts the matter
beyond question.
East of Hitchin the Great Chalky Boulder Clay ice-sheet
overrode the Chiltern escarpment, and no doubt it descended
the dip-slope normally in the direction of greatest slope. Within
the Tertiary country, however, it entered a region of irregular
relief in which it spilt up into various tongues, whose distribution
and motion were controlled by the form of the ground. In
short, it would seem that the Tertiary uplands ranging westward
from Danbury through Brentwood and across the Lea into
Hertfordshire acted as a vast composite fender holding the mass
of ice off from the region of the present Thames valley, but
allowing narrow ice-lobes, which must have had the dimensions
and appearance of true glaciers, to push through the gaps in
the ridge.
Thus, in the west, the ice split upon the north-eastern angle
of the Hertfordshire plateau, sending one branch south-westwards
down the Lea-Colne trough to Aldenham, and another southward
down the lower Lea valley. A minor tongue of the latter lobe
travelled westwards to Finchley, between the high grounds
of Hampstead and Barnet, and it seems probable that a similar
tongue continued down the Lea valley into the East London
neighbourhood, although, if so, all trace of it has been removed.
In Essex the prominent ridge of Epping Forest separated
the Lea valley lobe from that of the Roding valley. The latter
pushed southwards between the Epping Ridge and the high
ground of Lambourne End. Its outwash is represented by
the gravels of Woodford and Buckhurst Hill. In all probability
a small ice tongue insinuated itself between Lambourne End and
Havering, but of this no certain indication remains owing to