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There was a long period of time when the Irish landmass was lifted above sea level, meaning very limited sediment was deposited and extensive erosion of the exposed bedrock occurred. This is apparent when we look at the limestone bedrock and the karst landscape. Limestone is predominantly made of calcite, a mineral that dissolves in rain water due to its slightly acidic nature. When rain falls on limestone, it collects and starts to flow downhill, like on any other surface, but will eventually erode holes, known as sinkholes, into the bedrock. As more and more of the rock is dissolved, water collects and flows underground where it meets the water table, the level linked to sea level, at which all spaces in the bedrock are filled with standing water.

Sometimes, the underground water reappears at the surface, due to the local topography, as springs. The longer the bedrock is exposed to water, the larger the sinkholes and underground galleries are. When the water table drops due to a drop in sea-level, the underground stream carves a way further down and the chamber where it used to flow becomes an open cave. In these caves, rainwater still passes down vertically from the ground above, carrying some of the calcite that has dissolved in it. This calcite precipitates to form towers of calcite; stalactites, which hang from the ceiling of caves and stalagmites, which precipitate up from the ground.

We have seen how the last Ice Age eroded and exposed large parts of Irish limestone to the elements. However, most of the karst landscape of Ireland is much more developed than the amount of erosion over the last 11,700 years would have allowed.  In fact, the extent of the karst landscape indicates it is much older than 11,700 years and at the very least predates the Ice Ages that started just over 1 million years ago in Ireland. This is evidenced by some sediments found in the caves around Ireland which date back 35 million years ago to the Palaeogene. There are numerous karstic features to see and discover in the eastern half of our geopark project region, in particular around Cong and Clonbur. There, a unique system of sinkholes and springs explain the modern flow of water between Lough Mask and Lough Corrib. This spring complex, which consists of a number of swallow holes at the lake shore and 4 to 5 springs around the town of Cong, is among the largest in the world in terms of its flow. There are also a number of caves to be explored, such as the Pigeon-hole in Cong and Pollatoomary in Killawalla. Pollatoomary Cave holds the record of being the deepest explored cave in Britain and Ireland.

The limestone pavement that appears along the shores of the three Loughs within our geopark region display a great variety of erosion patterns from both rain and lake water. These are described by Michael J Simms in various papers ( Some of these erosion patterns were first described at Lough Mask and appear to be unique to the area. They are visible on the lough shores at Clonbur, Dringeen, Inishmacatreer, Oughterard and Moore hall.