Carboniferous limestone

The most widely-distributed sedimentary rock is Carboniferous limestones, the main rock across much of the central lowlands. In most places the limestones are overlain by materials deposited during or after the last glaciation. In a few areas, however, the cover is thin or non-existent and a distinctive limestone landscape has developed.

This landscape can be characterised by underground drainage, and by the exposure of limestone pavement, with its characteristic fissure into 'clints' and 'grikes'. Sometimes known as a karst or karstic landscape, this landscape is to be found in parts of Counties Fermanagh, Leitrim and Sligo in the north-west, east and south of Lough Mask

Lough Corrib

The waters of Lough Mask flow entirely underground towards Lough Corrib, rising in powerful springs at Cong. Found here is one of the most bizarre attempts to wish away the realities of limestone drainage. This is the famous ‘dry canal’ which was built in the 1830s to facilitate a Corrib navigation to Lough Mask. The canal proved useless, as the water sank through the limestone.

Courtesy of Ordnance Survey Ireland.

  and in other parts of east and south Galway, and most famously in the Burren district of north-west Co. Clare. The Burren covers over 200 and rises to over 300 metres.

Across much of the higher ground the limestone (which belongs to the older, or Lower, Carboniferous) is capped by younger, Upper, Carboniferous millstone grits and shales. On Slieve Elva (343 metres), streams run off the grits and shales, and then disappear into 'swallow holes' when they reach the limestone boundary. The water then travels underground, sometimes creating, over millions of years, great cave systems within the limestone. Some of these cave systems are 10-15 km in length, and have been explored and mapped over several decades by speleologists (people interested in the study of caves). Today, several of the more accessible caves, in the Burren and elsewhere in Ireland, have become major tourist attractions. Examples are Aillwee Cave

Aillwee Cave

Entrance to Aillwee Cave.

Copyright of Aillwee Cave.

  in the Burren, Marble Arch in Co. Fermanagh, Mitchelstown

Mitchelstown Cave

A photograph of Mitchelstown Cave, Co. Tipperary.

Courtesy of Mitchelstown Caves

 Co. Tipperary, Crag Cave near Castleisland, Co. Kerry, and Dunmore in Co. Kilkenny .
courtesy of the Geological Survey of Ireland
One of the most famous Burren swallow holes is at Poulnagollum. One of the best examples of underground drainage is in the lowland area immediately to the east of the Burren, where waters from the Gort area bubble to the surface on the shore near Kinvara. Other features of the limestone lowlands are the turloughs, or transient lakes, which appear and disappear with the rise and fall of the water-table. Examples of turloughs are widespread in the Ballinrobe area of south Mayo and in south Galway.

Karstic landscapes are of limited extent in Ireland today because the Carboniferous bedrock is usually overlain by glacial drift. For millions of years before the Ice Age, however, it is believed that Ireland was warmer than at present and that the land surface was exposed to prolonged weathering. Karstic conditions may then have been much more widespread, producing underground drainage and in places leading to the collapse of the overlying limestone. The unusual enclosed, depression at Carran in the Burren is 3 km x 2km in extent and may be the result of one such collapse. But collapses occurred in many places where limestone was exposed, on occasion with the result that the overlying rocks or soils were drawn down and preserved from later weathering. In this way a valuable but very patchy record has been preserved of Ireland's story over tens of millions of years before the Ice Age.

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