199
ON THE BOULDER-CLAY IN ESSEX.
By HORACE W. MONCKTON, F.G.S.
IF my readers will turn to the sheets of the Geological Survey
map which represent Essex they will see that nearly the whole
of Sheet 47, a portion of Sheet 48, and a considerable part of
the northern half of Sheet 1, are covered with the blue tint which
represents the Great Chalky Boulder-Clay, and few dwellers in
Essex can have failed to notice this curious chalky deposit which
forms so large a part of the surface of their county. That it was
brought into its present position by the action of ice is certain, and
I think it is pretty clearly proved that the ice which brought it was a
vast sheet of land ice of very great thickness which travelled over
the county from north to south. How far it travelled southward is
uncertain, but no Boulder-Clay is found south of the line of hills
which in Essex overlook the Thames valley. In its journey across
the county it necessarily encountered many obstacles, and in the
northern part of Essex it undoubtedly surmounted them all, for the
Boulder-Clay which the ice bore with it has been left covering the
ground on hill and valley; but in the south, towards the present edge
of the Boulder-Clay, this is not quite so clear, and the question I wish
to consider is how far this ice-sheet of the Glacial Period surmounted
the hills and ridges of southern Essex. I was led to investigate the
subject by noticing some diagrammatic sections in well-known papers,
which, whilst they correctly show the Boulder-Clay in the valleys
with the hills rising above it, seemed, perhaps unintentionally, to
imply that no Boulder-Clay ever occurs high up on the hills in
question. I allude to the diagrams by the late Mr. Searles V.
Wood ("Quart. Journ. Geol. Soc," vol xxxvi., plate 21, fig. 6,
line D; and vol. xxxviii., plate 26, fig. 33), and to that by Prof.
Prestwich ("Quart. Journ. Geol. Soc." vol. xlvi., plate 7, fig. 1),
taken together with the observation on page 145 of the same
volume. "On the hills near Epping, where they (the Westleton
Beds) attain a height of 350 or 400 feet, they form a thin bed of
sand and shingle, capping the London Clay, and quite apart from
the Boulder-Clay, which lies from 80 to 100 feet lower down on the
slope of these hills at Theydon Bois and North Weald."
The result of my investigation is to convince me that the sheet
of land ice, bearing its burden of clay and stones, did surmount the