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Wales has an irregular coastline with many bays, the largest of which is Cardigan Bay. Except for the narrow coastal plains, mainly in the south and west, the river valleys, and the lowlands of the Welsh Marches along the English border, Wales is almost entirely mountainous. The raised plateau of the Cambrian Mountains, which has an average elevation of 610 m (2,000 ft) and extends north-south through central Wales, occupies about two thirds of the country. Other major highland areas are the Brecon Beacons in the south-east, the rugged volcanic rocks of the Snowdon massif, in the north-west, and Cader Idris, in the west. Snowdonia contains the highest peaks in Wales; Mount Snowdon (1,085 m/3,560 ft), which gives the massif its name, is the highest point in England and Wales . The River Dee, which rises in Lake Bala, the largest natural lake in Wales, and flows through northern Wales into England and then the Irish Sea, is one of the country's principal rivers. The others are the Wye and the Severn, which both begin near Aberystwyth, flow eastward into England, and then turn south to empty into the Bristol Channel. In the south, many of the rivers flow through steep valleys, including the Usk, Teifi, and Towy. The main river of the north, apart from the Dee, is the Clwyd.