Later in the Carboniferous (c. 318 million years ago), the limestone was covered by darker sand and mud that later turned into shale (the lower "Clare Shales") and sandstone (the upper "Millstone Grit"). These layers reached a thickness of up to in north Clare. These top layers protected the underlying limestone from erosion for millions of years before being largely stripped away by glaciers, except in the south west, where they still extend from Doolin to Slieve Elva, Lisdoonvarna, Kilfenora and to the western shore of Lake Inchiquin. One "island" of shale is the hill Poulacapple, southwest of Ballyvaughan, where an upland moor has formed on top of the impermeable shale layers.