Here they have thrown up a great pebble barrier, the Chesil Beach, 28 km long and in places 200 metres wide and 14 metres high. Like a curtain wall of a castle this barrier has had profound effects on the area behind providing shelter from the fierce forces of the ocean.

Under the wall and stretching east from Abbotsbury for 13 km the long thin Fleet Lagoon provides a unique sanctuary for wildlife with its mouth opening into the bay, once called Portland Roads. Protected from the westerly gales by Chesil Beach the Roads were a shelter for sailing ships making their way up and down the Channel, eventually to be enclosed as Portland Harbour.

Movement of the seas has produced a grading of the beach's pebbles from west to east, small peas at the Bridport end to large fist size by Chesil. The seaward side of the beach is steep and bare, constantly scoured by the sea whilst a more gentle gradient on the sheltered side allows for some fauna to develop.

Once a danger for shipping, the scene of countless wrecks, wrecking and smuggling the beach is now mainly a nature reserve, for much of its length inaccessible to the public, but providing from overlooking high points a spectacular scene.