The rocks of the Pentland Hills were formed during volcanic eruptions around 400 million years ago. This low-level circuit of Black Hill takes us through a range of rock types and landforms that show how the Pentlands have been constructed and eroded. We will explore a variety of igneous rocks, formed by volcanic activity during the early Devonian period, around 400 million years ago. The rocks include andesite and microgranite, and they have been affected by a long history of deformation and uplift since they formed. We also get a glimpse beneath the volcanoes, to much older rocks formed on a vanished ocean floor.