14 GEOLOGICAL NOTES
However, my object is not to give a list even of the human
relics found in the excavations, but only to allude to them so far
as may be necessary to illustrate the formation of these recent
river-deposits, and consequently that of those which are older
and more obscure. Where a river has cut its way through com-
paratively hard rocks, it may leave terraces clear enough to be
traceable for a considerable distance marking its former position.
But this is not the case where the river's course, as in South
Essex, has been through a formation like London Clay. Then,
as we know, it may be possible to note a broad flat composed of
river-deposits which rises very gently northward, but it is im-
possible to trace a distinct terrace except for a few yards here
and there. As in a river-deposit the Mollusca usually vary but
little, it has naturally happened that geologists examining
sections in the older and more obscure fluviatile beds have often
dwelt mainly on the human relics or mammalian remains as the
chief guide to their relative antiquity, and have disregarded the
Stratigraphical evidence. For illustrations of the extraordinary
differences of opinion as to the older river-deposits of the Thames,
and the relative antiquity of various members of the series which
have thence resulted, see the chapter on the literature of the
River Drift in Mr. Whitaker's memoir on The Geology of London
and of Part of the Thames Valley, Vol. I., pp. 328-387.
Now in the case of these reservoir sections there can be no
controversy about their Stratigraphical position. No one can
assert that the beds at one spot in either reservoir belong to an
older series than those elsewhere simply because a pre-Roman
dug-out canoe has been found here, and tobacco pipes only there,
at the same level. The excellent sections, long but varying in
direction, have illustrated, as no other sections could have done
so well, how incessantly a river in its windings tends to change
its course, and yet how often a spot a few yards from a recent
or existing channel may have remained unmodified for centuries.
And consequently it becomes evident that in older river-deposits
there can be no legitimate excuse for assuming Stratigraphical
changes not only unsupported by, but contrary to, the available
Stratigraphical evidence, to account for apparent discrepancies
in the nature of the fossil evidence analogous to those presented
by the human relics found in the marshes of the Lea. In the
absence of evidence like that furnished by these reservoir excava-