High Quality Content by WIKIPEDIA articles! The Isle of Purbeck, not a true island but a peninsula, is in the county of Dorset, England. It is bordered by the English Channel to the south and east, where steep cliffs fall to the sea; and by the marshy lands of the River Frome and Poole Harbour to the north. Its western boundary is less well defined, with some medieval sources placing it at Flower`s Barrow above Worbarrow Bay. The most southerly point is St Aldhelm`s or St Alban`s Head. It is suffering erosion problems along the coast.