Julia's final walk takes her to North Wales, where two hundred years ago the great engineer Thomas Telford had to overcome seemingly impossible challenges in order to access the valuable slate industries of Snowdonia. In doing so he created a masterpiece of 19th century engineering - an aqueduct 126ft high and spanning a 1,000ft across the vale of Llangollen. To find out why it has become a world heritage site, Julia follows the cut of the Llangollen Canal, starting at the picturesque Horseshoe falls. Her six-mile walk takes her along the winding Dee valley, ending on the aqueduct that Telford described as 'a stream through the skies'.

• Julia Bradbury