SUBSIDENCE OF THE THAMES ESTUARY. 153
and destroys the balance hitherto established between the action
of river and tide. The normal functions of the stablilized river,
as regards meandering of the steam, widening of the valley,
erosion and deposition of material by the river and tidal scour,
are affected materially. There is a gradual encroachment of the
estuary westward and increased tidal action, which is felt to a
greater extent in the Lower Estuary than in the higher reaches
above East Tilbury, which are sheltered by the outcrop of chalk;
the increase of tidal scour causes more rapid widening of the
mouth of the river and the removal of material deposited where
land and sea currents formerly neutralised one another. New
phases are introduced, which must be taken into consideration
in estimating the extent of subsidence in the estuary and its
relation to Roman occupation sites. The more striking are:—
(a) Increased coastal erosion, from alteration in the force and
direction of ocean currents, in which subsidence plays an impor-
tant part. The loss of coast-line since the Roman period extends
over a large area; the Roman Forts of the Saxon Shore,
constructed in the 3rd-4th centuries, have suffered considerably
since the end of the Roman occupation in the 5th century;
Walton Castle has disappeared, as well as portions of Othona
(Bradwell-juxta-Mare), Regulbium (Reculver) and Rutupiae
(Richborough). The Viking fort of Hasten, at Shoebury Ness,
constructed in A.D. 894, has been affected in a similar manner.
(b) Widening of the estuary. Near its mouth the Thames
has cut away much of its northern shore, formed cliffs in higher
ground, and is separated only by a narrow ridge from the basin
of the Roach.
(c) Removal of large masses of material at one point and
deposition at another. Extensive land areas of the Roman period
have sunk and been covered by thick deposits of alluvium,
which form belts of marshland, protected by sea-walls, with
saltings outside. The channel of the Wantsum is closed and
the isle on which Richborough stood is surrounded by the marshes
of the Stour. Mr. A. S. Kennard informs me that the Cliffe
and Cooling Marshes are covered with estuarine clays with marine
shells and occupy an area that was a bay in Saxon times; the
former name persists in the stretch of the river called Higham
Bight.
(d) The extension of the estuary has shifted westward the