river of northeastern Wales, flowing mainly through Denbighshire but forming the border between Denbighshire and Conwy county borough at its mouth. It rises 7 miles (11 km) southwest of the town of Ruthin and falls about 1,200 feet (370 metres) as it flows 35 miles (55 km) through the Vale of Clwyd, past St. Asaph, to the Irish Sea at Rhyl. Beneath a covering of glacial material, relatively soft sandstones make up the floor of the vale, which forms a deep trough in the Welsh uplands.