SUBSIDENCE OF THE THAMES ESTUARY. 155
In most of these localities the evidence is complicated by
the intrusion of various factors, to which reference has been
made. A site has, however, been exposed recently at South-
church, Essex, near the northern shore of the mouth of the
Thames Estuary, where coastal erosion, tidal scour and the
vagaries of a river whose base-level of erosion is being lowered
may be definitely excluded. In a sheltered valley, well above
tidal influence, a settlement was formed in the Bronze Age or
earlier; a prehistoric causeway, dating possibly from before
500 B.C., presents in situ structural remains and pottery of the
prehistoric period, overlaid by Romano-British pottery of the
1st century A.D. There is definite evidence of subsidence
of the area after this date, when the floor of the freshwater valley
became a saltwater creek. The site of the causeway has never
been occupied by later settlers or exposed to the operations of
agriculturist or builder.
The geological and geographical setting may be considered
briefly. In the Pleistocene period the Thames flowed north-
east to join the Rhine, and the High and Middle Terraces
were probably continued along both banks. The right
bank extended from Sheppey parallel to the present coast
of Essex [28]. Remains of the left bank persist [29] and are
indicated in the Drift Map of the Geological Survey by an
almost unbroken chain of gravel patches extending from East
Tilbury to Bradwell, at the mouth of the Blackwater; flanked
on the east by beds of brick-earth and a few smaller scattered
gravel patches. The gravel patches have been formed by rivers
of post-Pleistocene date, which arose in the Essex plateau, cut
through and denuded the gravel terraces of the left bank and
passed across the plain of the old river to join the Thames and
North Sea. Denudation accounts for a gap between Fobbing
and Leigh, now occupied by marshland protected by sea-walls
and draining into creeks north and west of Canvey Island. Many
of these later rivers have tidal estuaries extending as far as the
gravel belt; the estuary of the Crouch has encroached westward
and annexed the middle affluents of the "Romford Stream,"
described by Mr. Holmes [30].
Intermittent advances of great ice-sheets in the Pleistocene
period separated the Thames from the Rhine, and its water
passed into the English Channel through the Straits of Dover [31].