82 CONTRIBUTIONS TO PLEISTOCENE GEOLOGY
a very deep channel in its lower course during this period, and the
evidence in the case of the Mardyke seems to be no less conclusive.
Firstly, at the mouth of the Mardyke valley a boring from the
level of the marsh passed through some 70 feet of alluvial beds.
Secondly, the Mardyke has cut a valley for itself of unknown
depth into the Chalk at Aveley, on the floor of which alluvial
beds repose. Thirdly, the valley of the Mardyke below Stifford,
though excavated for the most part in soft Tertiary strata, is of a
ravine-like character, pointing to very rapid erosion. Fourthly,
above the bend at Stifford the valley is wide and open, giving
rise to much fen-land.
Although from this evidence we think the main facts, namely,
that the present lower course of the Mardyke was excavated,
and that its valley in general suffered the maximum amount of
denudation in the period between the formation of the Lower
Terrace of the Thames and the commencement of Holocene
times, stand out sufficiently established, we confess that the
details of much of this latest portion of the history are obscure
at present, and that there remains much yet to be done before
all the minor problems connected with the history of this
interesting tributary can be regarded as solved. Chief among
these must be placed the origin of Orsett Fen, but this, dealing
as it does with a district beyond the limits of this paper, cannot
be discussed here,
Before passing to our conclusions there is a point connected
with the Lea Valley to which we wish to allude. Mr. T. V.
Holmes (Essex Nat., vol. xii., p. 224) has described evidence
of an old channel excavated in the London Clay to a depth of
57. feet at one point, as shown by the new puddle-trenches made
by the East London Water Company at Walthamstow. This
channel is filled with river gravel, sand, and silt, just as is the
case in the deep channel of the Thames. After noticing the
deep channel in the valley of the Cam, described by Mr.
Whitaker, Mr. Holmes says on p. 228—"It appears to me that
in the channel under the 'Lockwood Reservoir' we have, in all
probability, one of the same kind, and one also the result of
erosion in either Pre-Glacial or early Glacial times."
In our opinion there is very little or no connection between
the deep channel of the Cam Valley and that of the Lea at
Walthamstow beyond the self-evident fact that they are both the