The city is on the River Stour or Great Stour, flowing from its source at Lenham north-east through Ashford to the English Channel at Sandwich. The river divides south east of the city, one branch flowing though the city, the other around the position of the former walls. The two branches rejoin or are linked several times, but finally recombine around the town of Fordwich, on the edge of the marshland north east of the city. The Stour is navigable on the tidal section to Fordwich, although above this point canoes and other small craft can be used. Punts and rowed river boats are available for hire in Canterbury. The geology of the area consists mainly of brickearth overlying chalk. Tertiary sands overlain by London clay form St. Thomas's Hill and St. Stephen's Hill about a mile northwest of the city centre.