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Nant Ffrancon Pass, London to Holyhead Road
A5, Ogwen Valley, Conwy, Wales, UK
associated engineer
Thomas Telford
date  circa 1815
UK era  Georgian  |  category  Road  |  reference  SH643620
ICE reference number  HEW 460
The A5 main road through Nant Ffrancon Pass in Snowdonia follows the line of Thomas Telford’s London to Holyhead road, which was part of a transport link with Ireland. Despite the mountainous terrain, the road at this point was engineered to a maximum gradient of only 1 in 22. Many of its original masonry embankments, retaining walls, bridges and culverts remain in use.
The Holyhead to London road was constructed as a toll route and communications artery, and designed by Thomas Telford (1757-1834). Its Shrewsbury to Bangor section was completed in 1815-19 and includes the deep glacial valley of Nant Ffrancon in north Snowdonia.
Nant Ffrancon Pass is located west of the mountain called Pen yr Ole Wen on the stretch of road between Llandegai and Capel Curig, which follows the Afon Ogwen southwards, skirts the south side of Llyn Ogwen and then follows the Afon Llugwy eastwards downstream. The A5 has its summit at 312m above sea level at Pont Wern-gof (SH674605), some 500m beyond the eastern end of Llyn Ogwen.
Telford’s road replaced two older routes. Around 1791, a road was constructed on the west side of the valley, probably by Richard Pennant (1st Baron Penrhyn, 1737-1808), owner of the slate quarries at Penrhyn. In 1802, a turnpike way was built on the east side of the valley.
The 1791 road ran along the valley floor and climbed steeply up the mountains at the ends of the pass. The turnpike also had steep gradients, making it difficult to negotiate. Telford’s route overlaid parts of the turnpike, constructed on the east side of the valley some 6m above the valley floor. However, his road tended to follow rather than cross the ground’s contours and he ensured it had a maximum gradient of 1 in 22 to favour horse-drawn vehicles.
The route through the pass was one of the most hazardous sections of the London to Holyhead Road and was referred to by Telford as "the most dreadful horsepath in Wales". It was constructed with a carriageway between 9.2m and 12.2m wide and required substantial labour input, rock blasting and ongoing maintenance.
Describing the London to Holyhead project, Telford wrote: "This road, established through a rugged and mountainous district, partly along the slope of rocky precipices, and across inlets of the sea, where the mail and other coaches are now enabled to travel at the rate of nine or ten miles an hour, was indeed an arduous undertaking, which occupied fifteen years of incessant exertion".
During the 1990s, soil nailing was carried out in seven phases to strengthen Telford’s retaining walls in the Nant Ffrancon valley. The work was done in such a way as to preserve the original appearance of the structures.
Nant Ffrancon Pass is a Scheduled Ancient Monument and has a Grade II listed bridge at either end. Pont y Ceunant (SH632642) is on the old road to the north of the pass. Pont Pen-y-benglog (SH649605) lies on Telford’s road to the south, which at this point crosses the line of the former turnpike road as demonstrated by the arch of an earlier bridge beneath its north side.
The pass remains the easiest way to cross this part of Snowdonia by road. The present-day road incorporates a length of Telford’s revetment walling and ancillary works. His milestones, each weighing over one tonne, still mark the route.
Contractor: John Straphen & John Stanton
Research: ECPK
"Life of Thomas Telford, Civil Engineer, written by Himself" by Thomas Telford, edited by John Rickman, James & Luke G. Hansard & Sons, London, 1838
reference sources   CEH Wales

Nant Ffrancon Pass, London to Holyhead Road