The island is formed of Carboniferous Limestone and is often described as geologically a continuation of the Mendip Hills at Brean Down; however, the dip is at a different angle to that on Brean Down. On Steep Holm the dip is about 30 degrees to the north whereas at Brean Down it is 30 degrees to the south. There are some folds and fractures with dip angles up to 75 degrees created during the final phases of the Variscan orogeny near the end of the Carboniferous Period, 300 million years ago.