Geologically speaking, Glen Coe is the remains of an ancient supervolcano. The eruption happened about 420 million years ago during the Silurian period, and the volcano has long since become extinct. The glen is considered to be one of the best examples of a subsidence caldera, being the place where this phenomenon was first described. A caldera is a volcanic process in which a circular fault allows a section of crust to sink, leading to magma being forced to the surface at the periphery. This sinking allows layers of rock which would otherwise have been eroded to remain visible, can be seen in Glen Coe most strikingly in Devonian volcanic lavas on Bidean nam Bian, as well as at Sgorr nam Fiannaidh, An t-Sron, and in the bed of the River Coe below Loch Achtriochtan. Surrounding the caldera the rocks consists of schist and quartzite with a ring of granite marking the main ring fault.