 The main valleys between the hills are filled with alluvial deposits from the hills or sea . The county has many small rivers , most of which flow into the Bristol Channel . Many of the latter rivers now have <unk> ( the local name for a sluice ) on them to control the sea , but formerly they were tidal for some way inland . The main exception to this is the River Parrett , which still has a tidal bore . However the Chew and the Frome flow into the Avon which forms most of the northern county boundary with Gloucestershire . The Cale flows into Dorset through the Blackmore Vale , while the Exe flows into Devon . The ( Dorset ) Axe , the Culm and the Otter rise in Somerset but flow into Dorset . 
