SUBSIDENCE OF THE THAMES ESTUARY. 159
The Valley of Southchurch. The site under consideration
is in this valley, a shallow trough about two miles long and
nearly a mile broad. It extends from Samuel's Farm at the
north-east to Lower Southend at the south-west; the mouth is
between the Gas Works and the Halfway House Hotel. The.
floor is formed mainly by the eastern and western sections of the
Southchurch marshland, which indicate the positions of earlier
Eastern and Western Meres. (Fig. I.)
The valley passed through three phases; (a) Freshwater
Valley, well above tidal influence; the floor was mainly fenland
with scattered sedgy meres. This stage lasted until the Roman
occupation. (b) Saltwater Creek. Subsidence in the Roman
period was followed by flooding of the fenland floor with tidal
water and deposition of estuarine clays. The creek was co-
extensive with the marshland and reached the position of Thorpe
Hall Avenue. (c) Marshland, 8-9 ft. above O.D., after a
barrier had been constructed across the mouth of the creek to
exclude the tidal water. The valley was originally enclosed by
the gravel patch, except at its mouth; but on the north, east of
Holy Trinity Church, the encroachment of the basin of the
Roach has separated a gravel island at Bournesgreen Farm and
nearly reaches the marshland. At the head of the valley the
gravel is overlain by brick-earth beds continuous with those of
Shoebury and Wakering, which also extend south of the floor of
the valley nearly to its mouth.
The gravel island at Bournesgreen Farm, on the north of the
valley, is a plateau watershed between the basins of the Roach
and Thames, 200 yds. across, 20ft. thick, and its surface
281/2ft. above O.D. The west side has pockets of blue clay
with Pleistocene freshwater shells. The south side slopes
towards the valley floor and has patches of freshwater clays,
the remains of older beds of the stream. Before the develop-
ment of Thorpe Bay, the south limit of the valley was the escarp-
ment of the Southchurch Cliffs, 35ft. high, with a cap of gravel,
201/2ft. thick, overlying the London Clay. This gravel cap slopes
towards the valley floor, receives a covering of brick-earth and
presents patches of freshwater clays, as on the north. Thorpe
Hall Avenue extends from north to south across the valley, forms
the eastern border of the marshland and indicates the eastern
limit of the old saltwater creek and the earlier Eastern Mere.