IN PART OF NORTH-WESTERN ESSEX. 217
with the line of railway. The area is bounded to the north-east by
the Blackwater as far as Shalford, and a line drawn from that point
through Stebbing Mount to Dunmow High Wood completes the
northern limit of the area of observation.
Great changes have occurred at Braintree since Prof. Prestwich
visited the spot and drew his sections, presumably in 1849. Those
sections are now all concealed. New sections have, however, been
opened at higher, at the same, and at lower elevations, and these are
in the main confirmatory of those he drew. It is to be regretted
that the section in Black Notley cutting (of which he has given
such an instructive illustration, Plate vii., Fig. 6) is now covered by
talus, and there is no present equivalent in that neighbourhood ; that
is to say, no section at Braintree now shows the Westleton Beds
overlaid by the Middle Glacial Gravel. They are generally sharply
overlaid by Boulder Clay.
The section shown by the Black Notley cutting I have used as a
standard of comparison with other beds. These gravels and sand
vary greatly in their composition, but not to such an extent as to be
mistaken for the Glacial deposits of the district. Moreover, the
variations have proved valuable in cases where the section was small
and only one particular variety of sand or shingle was exposed. The
sand is sometimes ferruginous, sometimes of a rich yellow colour,
and in this case contains almost always abundant, though minute,
scales of mica. Sometimes it is quite white, like silver sand. It is
generally finely comminuted, and, with the pebbly gravel, contains but
little clay. The gravel is well described in Prof. Prestwich's paper
(l.c. p. 133). Generally it is made up about half of Quartz pebbles
and half of Flint, both well rounded. It has but very few sub-angular
flints, and none sharply angular ; and only a small proportion of the
stones in number are larger than a pullet's egg. Moreover, the
pebbles are enclosed in a matrix of yellow or iron-coloured sand,
which has but a small admixture of clay. In this respect the gravel
differs radically from the Middle Glacial Gravels of the district.
The Middle Glacial Gravels are well developed at Felstead and
Great Waltham, and generally just south of the area under con-
sideration ; and, however much these gravels may vary in more
widely separate localities, they certainly do not, for the few miles in or
near the tract under consideration, show a great amount of variation.
Cases occur where, as Prof. Prestwich points out, they are hopelessly
mixed up with the Westleton series ; but sufficient instances remain