 In prehistoric times the coastline of Somerset was very different from the present one , the sea level at the last glacial maximum being several hundred feet lower than today so that the Bristol Channel was almost non @-@ existent . The Bristol Channel has one of the largest tidal ranges in the world , up to 12 metres ( 39 ft ) at Burnham @-@ on @-@ Sea for example , behind only the Bay of Fundy and Ungava Bay in Canada . Normal high tide may be enhanced by between 3 metres ( 10 ft ) and 4 metres ( 13 ft ) during storm surges . This feature has meant that large areas of the county have been liable to flooding by the sea . Thus the present coastline is partly due a belt of marine clay at the coast and partly due to seawalls built to reclaim areas previously flooded at high tide . The coastline contains exposures of Devonian sediments and tectonics west of Minehead adjoining the classic exposures of Mesozoic sediments and structural features which extend eastward to the Parrett estuary forming cliffs along the coastline near Clevedon and near Minehead , with low sandhills near Burnham @-@ on @-@ Sea . There are sandy beaches mainly at Burnham @-@ on @-@ Sea , Brean and Weston @-@ super @-@ Mare . There are also storm ridges , salt marsh , and sand dunes . 
