The Isle of Sheppey is an island off the northern coast of Kent, England in the Thames Estuary, some 46 miles (74 km) to the east of London. It has an area of 36 square miles (93 km2). Sheppey is derived from the ancient Saxon "Sceapige", meaning isle of sheep, and even today the extensive marshes which make up a considerable proportion of the island provide grazing for large flocks of sheep. The island, like much of north Kent, comprises London Clay and is a plentiful source of fossils.