The limestones date from the Viséan age of the Lower Carboniferous, formed as sediments in a tropical sea approximately 350 million years ago, and compressed into horizontal strata with fossil corals, crinoids, sea urchins, and ammonites. Glaciation following the Namurian facilitated greater denudation. The result is that the Aran Islands are one of the finest examples of a Glacio-Karst landscape in the world. The effects of the last glacial period (the Midlandian) are most in evidence, with the islands overrun by ice during this glaciation. The impact of earlier karstification (solutional erosion) has been eliminated by the last glacial period. Any karstification now seen dates from approximately 11,000 years ago and the island karst is thus recent.