Hidden away above Bavelaw Marsh lies a rare exposure of mudstones and siltstones dating all the way back to the Silurian Period (435 million years ago) from a time when Edinburgh sat at the bottom of an ancient ocean called the Iapetus. These sedimentary rocks are particularly good at preserving fossils and a similar exposure in the valley of the North Esk is world renowned for studying marine creatures of that age. In Bavelaw, the sediments are cross cut by magmatic intrusions called dykes and are adjacent to the Black Hill, which is another igneous body thought to be a lava dome formed on the surface in early Devonian times (410 million years ago). To the south, the Silurian rocks are overlain by ‘Lower Old Red Sandstone’ conglomerate and the transition marks a significant change from deep ocean sedimentation to shallow high energy terrestrial conditions. The landscape was formerly carved out by glaciers, and rivers today follow rock weaknesses or glacial meltwater channels formed at the end of the last ice age. Examples of this are best represented above the marsh as you enjoy this Pentland walk.