At the centre of the range is an outcrop of Early Devonian granite, the Cheviot Pluton, which is surrounded by Silurian and Devonian arc andesitic lava flows, tuffs and agglomerates of the Cheviot Volcanic Formation. These are in turn intruded by a swarm of igneous dykes with a predominantly calc-alkaline chemistry arranged radially around the pluton. Both the pluton and the volcanic rocks owe their origin to the northward subduction of the oceanic crust attached to the former micro-continent of Avalonia beneath the Laurentian plate in the course of the Caledonian orogeny during the Ordovician and Silurian periods.