12 THE NEW RAILWAY BETWEEN UPMINSTER AND ROMFORD.
high and low ground alike, being found from the top of the Chalk
escarpment down to points not greatly above the level of the sea."
I may illustrate this statement by the following case. In the
valley of the Stort, about Sawbridgeworth, we find the Boulder
Clay forming the plateau on each side, and lower beds of Glacial
age exposed in the valley itself. Crossing the plateau in an easterly
direction, we come to the valley of the Chelmer, two or three miles
north of Chelmsford, in which lower Glacial Beds are also visible.
But, whereas the level of the Boulder Clay plateau about Saw-
bridgeworth is between 200 and 300 feet, on the Chelmer, near
Broomfield, it is between 100 and 200 feet, averaging nearly 100 feet
less. And, in addition to examples of this broad and general kind,
many might be brought forward illustrating the local variations of
small patches. In the Geological Survey Memoir on the Geology of
the Neighbourhood of Colchester, by Mr. W. H. Dalton, it is stated
that "between East Thorpe and Birch" (villages about two miles
apart) "the Boulder Clay lies in a hollow, either denuded in, or
formed by, the irregular deposition of the gravel.'' Now this hollow-
appears to have no connection with any existing system of drainage.
Many examples might be given, showing the very varied height
of the outlying patches of Boulder Clay found on or near the course
of a straight line drawn from Maldon to Brentwood : but it seems
unnecessary to do more than to allude to them, without attempting
to describe them in detail.
It may be useful, in conclusion, to give a brief summary of the
leading points in what I have written on this Hornchurch Boulder
Clay and the Thames Valley Beds.
In the first place, the discovery of the Boulder Clay beneath the
highest and oldest gravel terrace of the Thames valley system,
justifies the remarks made in a previous communication to the
Essex Field Club, as to the rashness of assuming that evidence of
the action of ice in some form is equivalent to evidence that the
deposit showing it dates from the Glacial Period. For if the Boulder
Clay is older than the Hornchurch gravel, it must be still more
ancient than the river deposits south of Upminster, or those of Erith,
Crayford, Ilford, or Grays. Secondly, the beds in which the Mam-
moth remains were found at Endsleigh Street seem to me to be simply
River Drift, of more recent date than the Hornchurch gravel ; and
the Mammoth there, as at Erith, Crayford, Ilford, or Grays, to be
in strata rightly termed post-Glacial, or, in other words, of later