There is an extensive network of caves, including Wookey Hole, underground rivers, and gorges, including the Cheddar Gorge and Ebbor Gorge. The county has many rivers, including the Axe, Brue, Cary, Parrett, Sheppey, Tone and Yeo. These both feed and drain the flat levels and moors of mid and west Somerset. In the north of the county the River Chew flows into the Bristol Avon. The Parrett is tidal almost to Langport, where there is evidence of two Roman wharfs. At the same site during the reign of King Charles I, river tolls were levied on boats to pay for the maintenance of the bridge.