The line of the railway was marked over the ridge and five vertical shafts were sunk. From the shaft bases, a horizontal drift was driven along the line. Although sufficient land had been purchased for the construction of a pair of tunnels, it was decided that only one would be built. The tunnel's bore passed through ground largely composed of Millstone Grit interspersed with argillaceous shale and softer sandstone on a gradient of 1 in 201, rising toward the east. Wires were suspended down each shaft from which the centre line was determined by means of a theodolite. The accuracy was such that the drifts lined up with less than three inches of error.