This site lies off the south coast of Dorset and contains numerous areas of reef in many forms, which exhibit a large amount of geological variety and biological diversity. Features of particular interest within the Studland Bay to Ringstead Bay area include a series of limestone ledges, up to 15 metres across, protruding from shelly gravel at Worbarrow Bay, which support a rich sponge and sea fan community; dense brittlestar beds (Ophiothrix fragilis)) on shale reefs extending from Kimmeridge; a unique reef feature, known as St Albans ledge, extending out over 10 kilometres offshore and subject to strong tidal action; and an area of large limestone blocks known as the “seabed caves”.
The Portland Reefs are characterised by flat bedrock, limestone ledges (Portland stone), large boulders and cobbles. On the western side of Portland Bill, rugged limestone boulders provide deep gullies and overhangs. Mussel beds (Mytilus edulis) are found to occur in very high densities on bedrock associated with strong currents to the southeast of Portland Bill.