A "slap" is a pass. Cauldstane probably refers to the wind that can
whip over the pass and when walked it feels like a very remote and
quiet place. As the route passes Harperrig Reservoir, the ruins of Cairns
Castle may be seen at the far end. This once belonged to Sir George
Crichton, Earl of Caithness, who charged himself with guarding and
warding the Cauldstane Pass through the Pentland Hills.