SUBSIDENCE OF THE THAMES ESTUARY. 163
to that from Walthamstow, Essex [44], a site which yielded
two iron socketed celts. The surface pottery of the islet road
and the presence of Helix aspersa indicate that the causeway
was in use as late as the early part of the Roman occupation.
During the whole of this period the site was in a freshwater
valley with sedgy meres, well above tidal influence, as indicated
by the shells, aquatic and terrestrial, and the elytra of beetles.
Since that date there must have been a considerable sub-
sidence of the area, as the bed of the mere is now 9ft. and the
upper surface of the causeway 51/2ft. below T.H.W. The exact
amount of subsidence is not easy to estimate, as the site is
11/2 miles from the mouth of the valley and 3/4 of a mile from the
nearest point of the estuary, but the minimum was 9ft.; the land
must have been several feet higher to exclude spring-tides ;
probably at least 15ft. and possibly more, or some of the Roman
sites on the estuary would have been submerged.
The subsidence was sufficient to admit salt water from the
estuary, which entered the valley freely and covered the site of
the causeway ; the fenland floor of the freshwater valley was
converted into a saltwater creek, co-extensive with the present
marshland. The date at which subsidence in the Estuary
commenced is uncertain ; Mr. Reader considered that there was
elevation of the coast at about the Roman conquest, followed
by subsidence to the present day ; it has been suggested that
subsidence began about the middle of the Roman occupation,
c. A.D. 200. The pottery of the sites to which reference has
been made is helpful and analysis of the potters' stamps should
enable us to arrive at more definite conclusions, but this has only
been done in a few cases, e.g. :
Red Hills. La Tene II.-III.; in a few cases Arretine ware or Romano-
British sherds were found.
Southchurch. Latest pottery of causeway was about 1st century
A.D.
Foulness. The pottery of the Belgo-British tumulus is early 2nd
century.
Leigh. The amphora is probably early 2nd century.
Canvey Island. Rheinzabern ware of 1st-2nd century has been
found.
Pudding-Pan Rock. The Lezoux ware is 2nd century.
Medway Marshes. The Upchurch ware is mainly 1st-2nd century.
East Tilbury. The pottery was La Tene III., with evidence of
Roman influence, and Samian pottery of 1st and 2nd centuries.