The Mendip Hills, of which Brean Down forms the most westerly part, are the most southerly Carboniferous Limestone upland in Britain. These rock strata were laid down during the early Carboniferous period, about 320-350 million years ago. Subsequently, much of northwestern Europe underwent continental collision throughout the late Paleozoic era, culminating in the final phases of the Variscan orogeny near the end of the Carboniferous period, 300 million years ago. This tectonic activity produced a complex suite of mountain and hill ranges across what is now southern Ireland, south-western England, Brittany, and elsewhere in western Europe. As a result of the Variscan mountain-building, the Mendip area now comprises at least four anticlinal fold structures, with an east-west trend, each with a core of older Devonian sandstone and Silurian volcanic rocks. West of the main Mendip plateau the Carboniferous Limestone continues in Bleadon Hill and Brean Down, and on the islands of Steep Holm and Flat Holm.