The north coast is gently shelving with low cliffs and long stretches of sandy beach, some sections of which, especially in the northwest, are muddy. The principal coastal feature is Bridgwater Bay, and the only important harbour is at the mouths of the Parrett. The main rivers in Somerset rise in the hills and flow northwards and westwards into the Bristol Channel; these are the Axe, Brue, Parrett and its tributaries, and Exe. The source of the River Axe is at Wookey Hole Caves in the Mendip Hills. It flows through a ravine and then west through the village of Wookey. It splits into two parts which subsequently recombine, and flows across the moors, through Lower Weare and to the south of Loxton. It passes between Uphill Cliff and Brean Down and reaches the sea at Weston Bay; it forms the northern boundary of the county.