At that time the seas around what was Ireland then were warm and shallow, supporting a vast array of marine life forms whose calcium-rich bones and shells fell to the seabed when they died. A combination of time and pressure from the sea-water and the compression of natural cementing agents fused these images of shell and bones together to form a sedimental rock- Lower Carboniferous Limestone- which was interspersed with beds of harder shale. These layers of permeable and less permeable rock provide the perfect environment for the preservation of fossils, the remains of marine organisms from ancient times, and can be seen today.
With the coming of the Ice Age the landscape was shaped and molded by the ice-sheets that scoured the rock surface depositing a fine layer of boulder clay and glacial erratics along the way. The soil however has since been eroded away by the forces of nature, and later by the arrival of farming. To explain away the existence of massive boulders of granite on a predominantly limestone landscape, folktales of an angry stone-throwing giant from the coastal area of Connemara served to form the basis of a lingering rivalry between these Irish-speaking communities, with the Connemara people earning the title of “Granites” echoing a perceived stubborn streak as well as pertaining to the geological landscape of coastal neighbors.
The landscape that greets today’s visitors is an expanse of bare limestone pavement, and must have once been connected to the mainland, as this is the same landscape as that of the Burren in north Co. Clare, the only two examples of karst landscape in Ireland.
Mythology has also played its part in the story of the formation of the Aran Islands, for the old annalists tell us of a distant time when only 3 lakes of any consequence existed in the whole of Ireland, one in Kerry, another in Mayo, and finally Loch Lurgen, described as a large lake between the present lands of Clare and Galway- the lake is supposed to have been the Bay of Galway over which the Atlantic burst through, leaving the three Aran Islands as the only defense line for Galway City against the respite of the sea.