72 CONTRIBUTIONS TO PLEISTOCENE GEOLOGY
a correspondingly higher level in relation to the sea, and from this
definite fact it is, we believe, possible within certain limits to
deduce what the relative level of the land was at any particular
stage in the history of the Thames, so far as it is represented by
the paired terraces.
III. THE HISTORY OF THE MARDYKE.
We now come to the consideration of one of the most
interesting and difficult of the numerous physical problems which
the Grays Thurrock area presents for our investigation, viz., the
history of the Mardyke from the High Terrace period to the
present day.
Mr. T. V. Holmes in the report of an excursion of the
Geologists' Association to this district described a section seen
in a subsidence which took place at a spot about one-third of a
mile nearer to Grays than Chalk Pit Farm (="Sugar Loaf
House" of the old one-inch Ordnance Map). This exhibited,
according to Mr. Holmes10:—
Red unstratified clay .. ,. .. 8 feet
Yellow sand .. .. .. 2 to 3 ,,
Gravel .. .. .. I to 2 „
Chalk
Further, he says :—
" In the great Chalk Pit west of the Grays and Stifford Road, which had
been visited the week before, many of the members had noticed at the northern
end of the pit evidence of a channel scooped out of the Thanet Sand. The
material seen at the subsidence had probably filled a northerly continuation of
this channel, which might have been that of a representative of the Mardyke,
flowing at a higher level than the present stream, and preserving between Stifford
and Grays the same direction it had above Stifford, instead of making a westerly
turn there to join the Thames at Purfleet, as did the modern stream."
One of us, before knowing of Mr. Holmes' opinion, had
arrived at the same conclusion with regard to the former course
of the Mardyke, but upon different grounds.
Mr. W. Whitaker in his Presidential Address to the
Geologists' Association11 in criticising Mr. Holmes' conclusions,
says:—
" I would rather class all this gravel as belonging to the old Thames,
thinking the Mardyke to be a more modern stream, not started perhaps till after
the formation of this High Terrace. We must remember that underground
dissolution of chalk may account for many irregularities in overlying gravel."
10 Proc. Geol. Assoc,, Vol. sii., pp. 200-201.
11 Proc. Geol. Assoc., Vol. xvii,, 89.