152 THE ESSEX NATURALIST.
Dr. R. E. Mortimer Wheeler [4] suggests that "Roman
London was established at, or only a short distance below, the
tidal limit at the time." The point at which tidal influence
ceased is of little importance, provided the river was navigable
for galleys above Tilbury. The position of Londinium did not
depend on tidal flow, but was determined by other considerations
—geological, geographical, commercial, administrative and
strategic—which have been discussed fully by Professor Haver-
field [5], Dr. Wheeler and other authors. In either case, sinking
of the flood plain was accompanied by a gradual encroachment
of the estuary westward and the tidal limit now reaches Richmond
(Spurrell) or Teddington.
The evidence for subsidence is more definite in the area
between Brentford and East Tilbury; it may be summarised
briefly thus:—
1. Brentford [6]. A Romano-British rectangular pile-dwelling, at
low-tide level, contained roofing-tiles, bricks and 2nd century-
pottery. Hallstatt pottery occurred at a deeper level.
2. Westminster (Spurrell). The Roman level is 4-5 ft. above O.D.,
at the Abbey site.
3. Southwark [7]. Seven square miles (4 x 21/2, E.-W.), are 4-5 ft
below, and only three limited areas between Deptford and Lambeth,
at Horsley Down, Bermondsey and St. George's Fields respectively,
are above, T.H.W. The Roman level is at O.D. or a little above it;
twenty sites are at an average depth of 91/2ft. below T.H.W.; there
must have been about 15 ft. subsidence since the Roman period.
A few sites at a lower level are associated with pile structures;
West Alpine methods were continued as a traditional art into the
Roman period and later, when low levels and wet soil made this
advisable; as at Southwark, the Walbrook, Brentford, Hedsor [8],
Cookham [9], Braintree [10] and Pickering [11].
4. Royal Albert Dock (Spurrell). The Roman level is 9 ft. below
the surface; 1-2 ft. below O.D.
5. Crossness (Spurrell, Codrington). The Roman level is 9 ft. below
the surface; 31/2-4 ft. below O.D.
6. Tilbury Dock [12]. The Roman level is 7 ft. below the surface;
about 2 ft. below O.D.
7. East Tilbury [13]. Roman burials and pottery are present beneath
the saltings for two miles; at 1—2 ft. below O.D. A Romano-
British settlement, with circular huts, was found on the foreshore
at 13 ft. below T.H.W.; it contained roofing-tiles, British pottery
showing Roman influence and Samian pottery of the 1st and 2nd
centuries.
Subsidence, whether localised in the estuary and adjacent
coast-line or part of a more general sinking affecting the east
coast of England, lowers the base-level of erosion in the Thames