The islands are composed from Old Red Sandstone and compressed slate, formed between 360 and 374 million years ago, as part of the rising of the MacGillycuddy's Reeks and Caha Mountains mountains ranges. This occurred during the Devonian period when Ireland was part of a larger continental land mass and located south of the equator. The region's topography of peaks and valleys are characterised by steep ridges formed during the Hercynian period of folding and mountain formation some 300 million years ago. When the Atlantic ocean level rose, it created deep marine inlets such as Bantry Bay and left the Skelligs detached from the mainland. The rock is highly compressed and contains numerous fracture lines and jointing. As a result of erosion along a major north-south-trending fault line containing bedrock much more brittle than that on surrounding areas, a large part of the rock broke away, resulting in Christ's Saddle, the depression between the peaks. The island's rock is deeply eroded through exposure to wind and water.