191
NOTES ON THE GLACIAL FORMATION NEAR
CHELMSFORD.
By HORACE W. MONCKTON, F.G.S.
[Read at the Field Meeting on July 11th, 1891.]
THE sections which we shall see this afternoon1 illustrate very well
the Glacial formation of this part of Essex. It consists of—
(1) The Great Chalky Boulder Clay.
(2) The Glacial Sands and Gravel.
In Norfolk these sands and gravel are underlain by a second
Boulder Clay; but here they rest directly on the London Clay, a
marine formation of Eocene age, very much older than the Glacial
Period.
The origin of the Boulder Clay has given rise to a great deal of
controversy. At one time it was supposed to be due to a series of
great waves raised by hurricanes and storms which swept over the
continents, carrying mud and stones of all sorts with them; but that
theory has long been abandoned, and all geologists now, I think,
agree that both the Boulder Clay and the materials of which the
accompanying sands and gravel are formed were brought into this
part of the country during the Glacial Period or Great Ice Age by
the agency of ice. There is, however, a great difference of opinion
as to the manner in which the ice did the work of transport. Sir
Charles Lyell favoured the view that the Boulder Clay was formed of
mud and stones melted out of floating ice when nearly the whole of
England north of the Thames and Bristol Channel lay submerged
beneath the sea ("Antiquity of Man," 4th ed., 1873, p. 273) ; but
many geologists now attribute the transport of the material of which
the Glacial beds are formed to the agency of land-ice, either in the
form of a vast sheet which covered a great area, or to more or less
local glaciers. The Chalky Boulder Clay is supposed, according to
the land-ice theory, to have been pushed or drawn along under the
ice, or to have been carried enclosed in the ice and deposited where
we now see it when the ice melted ; whilst the sands and gravels are
supposed to be due to glacial streams or rivers flowing over, through
1 The sections visited were in the gravel pits at Rainsford End, Writtle Mill, and Roistons,
all in the neighbourhood of Chelmsford and Writtle. See the account of the Excursion,
post.