The Great Stour Project - River and Coastal Flooding

The geomorphology of the English Channel

Joint Chatham House and St. Lawrence College Field Day: 26 April 1999

The Straits of Dover are a relatively young feature when compared with the Western Channel and the North Sea. They correspond to an anticline, the axis of which runs from east to west from roughly the position of Hastings to Boulogne on the other side of the Channel. Surface rocks belong typically to the Cretaceous period though on parts of the Channel floor and in the Boulonnais of Northern France, there are exposures of underlying Jurassic strata.

On either side of the Wealden anticline lie the structural basins of the North Sea and the Hampshire Basin which links with the Dieppe Basin between Sussex and the Normandy Coast. The structural evidence and the identification of many raised marine erosion services along the South Coast tends to suggest that the English Channel was an arm of the Atlantic Ocean throughout most of the Quaternary. However, there were periods during the Pleistocene when sea level was considerably lower than it is today.

During the Devensian, the sea almost certainly abandoned the whole of the English Channel corresponding to a fall in sea level of least 100 metres. As a result the shoreline retreated to a position between Cornwall and Brittany, with the temporarily emergent Channel floor being drained to the West by a Seine-Solent river system.

At times the discharge of this system was probably very great because of periodic inputs of large volumes of water which escaped southwards along the line of the Straits of Dover from extensive pro-glacial lakes dammed up in the southern North Sea Basin in front of the Scandinavian ice sheet. Since these lakes were fed by both the Thames and Rhine river systems the effective catchment of the Seine-Solent River or Greater Seine was enormous.

The extremely high discharges achieved by the English Channel drainage system during the Wolstonian and Devensian caused a marked incision. This is reflected in a well defined palaeo-valley system running down the floor of the Channel with a number of deep enclosed basins. For example the rock floor of the Sussex Ouse at Newhaven is a 29.5 metres below current sea level.

The retreat of the Devensian ice sheets resulted in a marine transgression with the sea level rising by over 100 metres in the last 14,000 years. Approximately 45 metres of this rise have been accomplished during the last 10,000 years, which is generally known as the Flandrian or postglacial transgression.

During the early part of the Flandrian transgression sea level change is considered to have been extremely rapid, rising from below -100 metres at a 14,000 BP to reach about -20 metres by 8000 BP.  As a consequence, the sea advanced quickly up the Channel and through the Straits of Dover by 9600 BP achieving the final separation from the Continent in about 8600 BP.