The 'Mountain line' is the second of the three imaginary lines crossing the Vale of Glamorgan on whose intersection the Tinkinswood Burial Cairn is located, the others being the ‘Rivers’ line and the ‘Coastal’ line.

The ‘Mountain’ line is 4,000 poles in length (one two-thousandth of the earth’s circumference). It runs roughly north-south from Foel – a hilltop near Llantwit Fardre - at map reference ST 073857 to Cold Knap Point in Barry.

Tinkinswood lies on this line exactly 1,500 poles* from Cold Knap Point and 2,500 poles from Foel. This ratio of 3:5 is exactly the same ratio as we observed with the 8,000 pole ‘Rivers’ line, running between Col-Hugh Mouth and Usk Mouth.

Moreover, unlike the ‘Rivers’ line, the ‘Mountain’ line offers precise measuring points – the top of a hill and the end of a small headland jutting into the sea. And although the trigonometrical point on the Foel hill top is not itself very high (it is 173 metres) it affords a commanding view of the countryside below and to the south of it. For this reason today the hilltop hosts an ugly modern telecommunications mast.

As was described in the section on common features of the burial sites in the Vale of Glamorgan the direction of the hill-top with which each site is associated is frequently indicated by a further ancient site close by. Thus 200 poles to the north of Tinkinswood, on the ‘Mountain’ line, lies St Nicholas Church – a fine mediaeval church – which ‘points’ the way to the Foel hill-top.