THE NEW RAILWAY BETWEEN UPMINSTER AND ROMFORD. 13
date than the Chalky Boulder Clay; the only standard of age that
can be taken without introducing confusion.6
On the other hand, I pointed out that the absence of definite,
traceable terraces was invariably to be found where a river cuts
through soft, incoherent strata ; and that it implied no peculiarity
in the formation of the river valley. That the general evidence all
pointed towards the view that the highest terrace was in this case,
as usual, the oldest; and that, in default of special evidence point-
ing yet more strongly another way, the only rational course was to
hold this view. That the low level of the Hornchurch Boulder Clay
necessarily implied merely deposition in a narrow valley, or hollow
with a direction at right angles to that of the present Thames.
That there may have been at the time a valley, possibly coinciding
here and there, more or less, with that of the Thames ; but that we
had no evidence of it and its deposits, the present valley and its
beds being post-Glacial. And that the Essex Boulder Clay lies on
a very uneven surface, and at various levels.
In short, I maintain that the evidence now available clearly
points to the conclusion that the River Drift of the lower Thames,
with the Mammoth and other remains contained therein, is post-
Glacial. Absolute proof is, of course, unattainable. But it seems
to me that this conclusion can be discredited only by the discovery
of some special evidence pointing yet more strongly in another
direction. And of this we have, at present, no sign.
Notes.
A most interesting example of the occasional coincidence and
frequent variation between the pre-Glacial and post-Glacial channels
of an English river, is given by Sir Andrew Ramsay in his "Physical
Geology and Geography of Great Britain" (edit. 5, p. 531). It is
that of the Wear, in Durham. A letter from Mr. H. H. Howell
(who superintended the Geological Survey of the Newcastle coalfield)
to Sir A. Ramsay thus describes the changes that occurred :—
" The pre-Glacial valley runs nearly north and south from
Durham to Newcastle. The river Wear, instead of following this
old valley, meanders about, winding in and out of it, and at Durham
cutting right across it, and passing into the sandstones of the Coal-
measures, through which it has cut its way in a narrow gorge. At
Chester-le-Street, half-way between Durham and Newcastle, the river
6 Of course I by no means assert that the Mammoth is necessarily to be found only in post-
Glacial Beds. I see no reason why it should not be pre-Glacial as well. Each case must be
decided on its own evidence.