The area is formed from a thick succession of sedimentary rocks, largely limestones but also including sandstones, mudstones and siltstones. All of the solid rocks exposed at the surface are of Carboniferous age though they are underlain at depth by Old Red Sandstone of Devonian age which in turn overlies rocks of Lower Palaeozoic age. None of the pre-Carboniferous rocks is seen at the surface in the area. The limestones, which date from the Visean stage of the early Carboniferous, formed as sediments in a tropical sea approximately 325 million years ago. The strata contain fossil corals, crinoids, sea urchins and ammonites. This bed of limestone is up to 800 metres thick. In the north and west it lies on a shelf of Galway granite which supported the upper layers, preventing shifts like those that created the "twisted" hills Knockanes and Mullaghmore. The limestone also extends below Galway Bay out to the Aran Islands and to the east into the Gort plain.