The hills are made up of layers of igneous rocks from the Carboniferous[8] which show differing resistance to erosion.

In the volcanic sequence of the hills there is hard basalt lying over trachytic lavas.

The flow of ice and meltwater along the strike of the ridge has eroded furrows and channels.

Around the village of Athelstaneford there is a rock basin which has been partly gouged out of the softer basalts.

[9] The hills are designated by Scottish Natural Heritage as a Geological Conservation Review site (nr.