The Pontcysyllte Aqueduct was built by Thomas Telford in 1805 and is the largest aqueduct in the United Kingdom; it carries the Llangollen Canal over the River Dee and is a World Heritage Site, being considered a masterpiece of civil engineering. The Clwydian Range and Dee Valley constitute an Area of Outstanding Natural Beauty, one of just five in the whole of Wales. Denbigh, Colwyn and Ruthin are historic towns and Llangollen hosts the Llangollen International Musical Eisteddfod in July each year.