Nant Ffrancon itself is a steep-sided glacial valley dropping to Bethesda between the Glyderau and the Carneddau. The valley starts in Cwm Idwal, carrying water from Llyn-y-Cwn through Twll Du and Llyn Idwal to join the Ogwen Valley below the Ogwen Falls on Afon Ogwen. Unlike Lord Penrhyn's road, which largely followed the valley floor, Telford carved much of his road out of the north-eastern slopes of the Nant Ffrancon, thereby encountering difficulties in construction and future maintenance. But this enabled Telford to observe a maximum grade of 1 in 14 along the whole route from London to Holyhead in order to facilitate the operation of horse drawn mail coaches throughout.