SALT-MARSH ACCRETION AND SEA-LEVEL MOVEMENT IN THE INNER SEVERN ESTUARY, SOUTHWEST BRITAIN - THE ARCHAEOLOGICAL AND HISTORICAL CONTRIBUTION

The sea defences of the inner Severn Estuary may be dated partly by reference to historical sources and partly by the detection through field-walking of early settlements on the reclaimed wetlands. Largely as the result of an intensive programme of levelling from reclaimed to active tidal marshes across dated defences, it is shown that relative sea-level (c. mean high-water spring tides) has risen in the area by at least 1.3 m since the later Roman Period. The rise since c. 1945 is given by a geochemical method which exploits the presence of metal pollutants in the deposits of the active marshes. A marked acceleration in the upward movement of the relative level is suggested by the assembled data. The minimum average rate of rise is apparently 0.40 mm a-1 between the later Roman Period and the Medieval Period, 0.79 mm a-1 from then to the start of the Modern Period, 1.49 mm a-1 through the Modern Period to c. 1945, and 4.65 mm a-1 since c. 1945. Depending on the extent to which the Severn Estuary has experienced marine transgression over the period covered by the measurements, the relative rise of mean high-water spring tide level to the present could amount to about 1.75 m.