Wales is located on the western side of central southern Great Britain. To the north and west is the Irish Sea, and to the south is the Bristol Channel. The English counties of Cheshire, Shropshire, Herefordshire and Gloucestershire lie to the east. Much of the border with England roughly follows the line of the ancient earthwork known as Offa's Dyke. The large island of Anglesey lies off the northwest coast, separated from mainland Wales by the Menai Strait, and there are a number of smaller islands.