THE NEW RAILWAY BETWEEN UPMINSTER AND ROMFORD. 11
case the Boulder Clay ended a short distance northward of the
edge of the River Drift, and the probable relations of the two
formations could be but matter for speculation. The most southerly
point at which the Boulder Clay appears at the surface, in the
Romford district, is at Maylands, about three miles north of the
Hornchurch cutting. And it is noticeable that at Maylands the
Boulder Clay comes down to a level considerably below the 200
feet contour-line, though almost all of it, in the immediate dis-
trict, lies above that height. It was, therefore, suggested during
the discussion on my paper at the Geological Society last March,
that the Hornchurch Boulder Clay, being at a level still lower than
that at Maylands, pointed towards the probability of the pre-Glacial
age of the Thames valley system. But this is, I think, to exaggerate
greatly the significance of the low level of the Boulder Clay at
Hornchurch. All that it really does tend to show is, that there
once was probably a hollow or valley parallel with that of the Ingre-
bourne, having a direction from north to south, or nearly at right
angles to that of the present course of the Thames, and that the
Boulder Clay more or less occupied this hollow. There may have
been at the time a valley to some extent coincident, here and there,
with that of the present Thames; but its deposits remain unknown
to us.5 Of those which we find in the present valley system, the
oldest terrace—as we are entitled to assume that at Hornchurch to
be, in the absence of special evidence to the contrary—is manifestly
post-Glacial in the sense of being later in date than the Chalky
Boulder Clay. And this, as I have already remarked, seems to me
the only sense in which the term can rightly be used in south-eastern
England.
I have said that we may easily exaggerate the significance of the
low level of the Boulder Clay at Hornchurch. Where, as in northern
Essex, we see Boulder Clay forming a continuous plateau ; or one
the continuity of which is broken only by the valleys of streams in
which lower beds are exposed, we find the base of the Boulder Clay
at the most various levels. In the Geological Survey Memoir,
on Sheet 47 (which includes north-western Essex with portions of
adjacent counties), we find the following general statement:—"It
is evident the clay was not formed in fragments as mapped, but in
one continuous sheet. It has been cut through in post-Glacial
time by the present valleys ; but, with this exception, it spreads over
5 See Notes at end of paper.