192 NOTES ON THE GLACIAL FORMATION NEAR CHELMSFORD.
or under the ice, and to floods caused by the melting of greater or
lesser portions of the ice.
The materials of which the gravel is composed are—
(1) Flint pebbles more abundant at Rolstons than at Rainsford
End, probably derived for the most part from pre-glacial
pebble gravel, such as that which still caps the hill at
Writtle Park.
(2) Sub-angular flints, the brown colour of which shows that they
have not been derived directly from the Chalk, but from
older gravels.
(3) Black flints, not much worn or rolled, derived from the Chalk.
The nearest point at which the Chalk crops out is twenty-
five miles distant. These flints have, therefore, travelled
at least twenty-five miles, and must have been carried
embedded in ice, for they would have been rolled and
water-worn had they been brought all the way by water.
(4) Pebbles of quartz of a white or pink colour, derived either
from older pebble gravels or from the Triassic beds.
(5) Pebbles of red and white quartzite from the Triassic beds of
the north. The nearest point at which these beds now
reach the surface is near Leicester, ninety miles N.W., so
that these pebbles have been carried at least ninety miles,
no doubt by ice. The presence of these pebbles in a
gravel proves it to be either Glacial or formed of debris
from Glacial Gravel.
(6) Blocks of white quartz and of various old rocks, and frag-
ments of Lias, Oolite, &c., probably nearly all brought by
ice from the north.
(7) Small pieces of chert, originally from the Lower Greensand,
but derived at secondhand from older gravels.
The Chalky Boulder Clay is here mainly composed of Chalk, and
contains pebbles of chalk, chalk flints, pebbles, and fossils from
Oolitic or Liassic strata, the whole having come from the north.
The thickness of the Glacial beds is very variable, and they rest
on a very uneven surface of London Clay, Chalk, &c., often filling
deep Pre-glacial valleys, as at Littlebury, near Saffron Walden, for
instance (see Whitaker on "A Deep Channel of Drift in the Valley
of the Cam, Essex," Quart. Journ. Geol. Soc, 1890, p. 333, and
Essex Nat., vol. iii., pp. 140-142, and vol. iv., p. 117), or banked
up against Pre-glacial hills which in places rise above them, as at
Writtle Park. There are some patches of gravel and brick-earth in
this neighbourhood which are believed to be newer than the Glacial
beds, but in none of them, nor, indeed, in any beds newer than the