The hills are formed of Carboniferous basaltic lava and ash on sandstone.

On part of Wester Craiglockhart an exposed upper lava deposit about 30 metres thick features a central area of well-formed columns.

When this lava crystallized, a series of hexagonal cracks produced the columnar structure.

The valley took its present form over 17,000 years ago when an ice sheet covered the area.

Meltwater from the Water of Leith, then dammed by ice, cut the channel.