 Aligned on a south-west to north-east axis, it measures in length and reaches at its widest point.The Colonsay Group, which takes its name from the island, is an estimated sequence of mildly metamorphosed Neoproterozoic sedimentary rocks that also outcrop on the islands of Islay and Oronsay and the surrounding seabed.The sequence has been correlated with the Grampian Group, the oldest part of the Dalradian Supergroup.