Geoscience Reference
In-Depth Information
years ago, the advance of the Anglian ice from the north extended across the course of
this ancestral Thames, locally damming the river and deflecting it progressively south-
wards to finally take up its present path. The hill ridges of south Essex, the Valley of
Romford and the estuaries of the Blackwater and Crouch are all likely to have been
shaped during episodes when the Thames or its immediate tributaries flowed in that
direction.
Coastlines and sea-level rise
The coastline of the Thames Valley area is one of the youngest features of the land-
scape, since it has been profoundly affected by the dramatic rise in sea level over the
last 10,000 years. At the end of the most recent cold phase of the Ice Age, the River
Thames and its tributaries flooded seasonally over wide, braided floodplains. The rise
in sea level flooded the Channel, turning the lower river valleys of the Thames and its
tributaries into tidal estuaries. The sea level did not rise continuously, however, and
small drops in sea level are marked by layers of peat in the sediment. The peat lay-
ers are the remains of plants which colonised the exposed mud flats when sea level
dropped, only to be buried by silt and clay when the sea level resumed its rise once
more. The last peat layer dates from the third or fourth century AD, when sea-level rise
destroyed several Roman forts built on the coast.
Away from the estuaries, most of the coastline consists of large expanses of mud
flats and salt marshes, although much of the original marsh has been drained for ag-
riculture. The coast between Southend-on-Sea and Dengie (30 km to the northeast) is
typical: salt marsh is separated from intertidal mud flats by
chenier ridges.
These are
raised ridges almost totally composed of seashells, which can be up to 3 m high and
25 m wide. They mark the transition between land and shore, since they are formed
when shells are thrown up onto the salt marshes by storm-whipped waves. The ridges
are moving landward at up to 8 mm a year due to continuing sea-level rise in this area.
Sea-level rise poses an important problem for the London area. Due to a combina-
tion of subsidence and global sea-level rise (estimated at ≈1-2 mm a year since 1900),
levels are rising in the Thames estuary by about 60 cm a century, relative to the land.
After the 1953 flood of the east coast and Thames Estuary, when some 300 people
were killed, it was decided to build the Thames Barrier (Fig. 185). This is situated at
Woolwich and has six moveable gates which can be raised in minutes if needed. It
was built between 1972 and 1984, and was designed to be effective until at least 2030.
Many longer-term solutions to protect London are also being considered. One recent
proposal is for a 16 km (“ten-mile”) barrier from Sheerness in Kent to Southend in
Essex, in order to protect a much more extensive section of the Thames Estuary (Fig.
180).
