SUBSIDENCE OF THE THAMES ESTUARY.
157
on them or in their immediate vicinity. They resemble in this
respect the gravel terraces of the Thames, Lea, Colne and the
Fenland rivers draining into the Wash; they took the place of the
chalk, oolite and limestone hills occupied by prehistoric man in
many counties. The gravel areas were relatively small and
could support only a scanty population and successive civiliza-
tions occupied the same sites. Deforestation was impossible with
the labour and toolsa vailable, and was hardly attempted before
the Roman occupation [33]. There was then some lateral
extension into the woodland by deforestation at the periphery
of the settlements, and these old sites were linked up with new
riverain settlements and forest clearings in the Saxon period.
A gravel patch, cut off by the Prittle Brook, extends to Leigh
and the shore of the estuary at Southend and is prolonged as a
littoral zone through Southchurch to South Shoebury. It is
over 90ft. high at the west, but slopes down to sea-level at the
east; it represents the High and Middle Terraces altered by
denudation and re-deposition during the changes in the course
of the river and formation of new streams; associated with it
are extensive beds of brick-earth. This patch has yielded
Pleistocene freshwater shells at Southchurch [34] and Shoebury
[35], mammoth remains at Prittlewell [36] and Southend, and
tools of Lower Palaeolithic man [37] at Shoebury, Leigh, Southend
and Prittlewell.
This littoral zone has been much eroded on its northern side
by the encroachment of the lower part of the basin of the Roach.
The southern side belongs to the basin of the Thames, which
cut away part of the patch to form the old Southchurch Cliffs,
but left in it the river-heads of three tributaries, "Porter's
Creek," the "Valley of Southchurch," and a valley at South
Shoebury. Small steams, issuing between the gravel and
London Clay, excavated shallow valleys in the gravel patch and
on leaving it passed through low lands, now submerged, to join
the Thames. This littoral zone has the dominant position in
the prehistory of this region ; on the northern shore of the
estuary it offered the first landfall to the continental immigrant
entering the Thames corridor and was the only possible site for
settlement below East Tilbury. It has yielded evidences of the
Neolithic Age, Beaker period, Bronze Age II. and III., West
Alpine, Hallstatt, La Tene and later cultures.