Geology
The islands' geology is mainly karst limestone and is thus closely related to The Burren in Co. Clare (to the east), not the granites of Connemara to the north. This is most obvious in the construction of the walls around the fields.
The limestones date from the Visean period (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 phase 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. So any Karstification now seen dates from approximately 11,000 years ago and the island Karst is thus recent.
Solutional processes have widened and deepened the grykes of the limestone pavement. Pre-existing lines of weakness in the rock (vertical joints) contribute to the formation of extensive fissures separated by clints (flat pavement like slabs). The rock karstification facilitates the formation of sub-terrainean drainage.
Huge boulders up to 25 metres (80 ft) above the sea at parts of the west facing cliffs have been shown to be sometimes an extreme form of storm beach, cast there by giant waves that occur on average once per century, though more are the consequence of glacial erratics.
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