52 THE ESSEX NATURALIST.
(6.) No foundations of any house or fixed kiln have been
located at any of the 50 test holes dug on the four sites in
the Island, though fragments of Roman tiles have been
frequently met with in the waste tips of the domestic sites ;
wood structures, if they ever existed, must have long since
been swept away or have perished by decay.
(7.) Land in occupation as arable land at Northwicke
in 1557 A.D. (see Morant), 70 years prior to the erection of
the sea-wall by the Dutch, and which must then have stood
at a level at least 13 ft. above O.D., now stands at 11 ft.
above O.D.
The facts summarised above are consistent with the occupa-
tion at successive periods of a burnt earth surface subsiding at
an average rate of about 8 ins. per century—a figure confirmed by
independent evidence (Ibid, pp. 283-290). Subsidence may have
occurred in a series of short steps, and may have been inter-
rupted by periods of arrest or even by slight reversal of move-
ment, but the results yielded by excavation afford no support
to the view that subsidence has been catastrophic at any period
subsequent to the close of the Romano-British period, or that it
had completed itself prior to the arrival of the Dutch in the 17th
century.
At an early date in the enquiry it was recognised that the
stratification of the pottery—apart from that of mediaeval date
—has little or no relation to chronological sequence. Broadly
speaking, it may be said that the pottery of the Romano-British
period of occupation was derived from two contemporary
sources:—
(a) A finer kiln-fired type, manufactured elsewhere and
brought on to the sites for domestic use, and
(b) Crude red pottery, manufactured on the sites.
Some of the more interesting problems connected with the
classification of the fragments find illustration in the section that
follows.
STRATIFICATION OF POTTERY—INTERCALARY
CLAY.
At an early date in the exploration of the Red-Hill mounds
on the mainland further north, workers were puzzled by the
presence of layers of intercalary clay of varying thickness—
Mr. Francis W. Reader considered this clay to be tidal