6 THE NEW RAILWAY BETWEEN UPMINSTER AND ROMFORD.
have received a signal illustration in the Boulder Clay of the Horn-
church cutting. For, in the absence of special evidence to the
contrary, we may rightly assume that the gravel covering the Boulder
Clay at Hornchurch, which forms part of a higher terrace than that
traversed by the railway south of Upminster, belongs therefore to an
older one. Hence, if the Boulder Clay is evidently older than the
Hornchurch gravel, it must rightly be considered to be still more
ancient than the gravel nearer the Mardyke. Locally, the only
sound test of the Glacial or post-Glacial age of a bed is the nature
of its relation to the Chalky Boulder Clay of the district, and to
attempt to employ any other is but to introduce confusion. Of
course, the age of the Chalky Boulder Clay of Essex as com-
pared with that of any given deposit of the Glacial Period in
Lancashire, Scotland, or elsewhere, may rightly be a question for
discussion and speculation. But in Essex and Middlesex the
only standard of comparative age is that furnished by the local
Boulder Clay.
It is then evident that if we may rightly conclude that the Horn-
church gravel is older than that at less elevation between Upminster
and the Mardyke, we are also justified in deciding that it is older
than the river deposits which occur at a lower level at Grays and
Ilford, or at Erith and Crayford on the Kentish shore.
It is also obvious, that if we compare the Hornchurch gravel with
fluvial deposits fifteen or twenty miles higher up the course of the
river, we should expect to find its equivalent in beds with an eleva-
tion as much greater than that at Hornchurch as the fall of the river
per mile, when the gravel was deposited, would indicate. If, for
example, the average level of the Hornchurch gravel should be 100
feet, then, supposing a fall of one foot per mile, the equivalent terrace
fifteen miles higher up would be 115 feet, and so on.
Thus the natural inference is that as the Hornchurch gravel is
older than the various river deposits of Grays, Ilford, Erith, and
Crayford, and the Chalky Boulder Clay is evidently older than the
Hornchurch gravel, the fossil remains found in the river deposits of
all the places just named are rightly considered to be post-Glacial.
The probable age of the Chalky Boulder Clay, as compared with that
of beds of Glacial age in Wales, Scandinavia, or elsewhere, is, as I
have already remarked, a wholly distinct question. In what is
termed the Glacial Period, glacial beds may well have been in process
of formation in the north of Scotland long after they had ceased in