Approximately 40% of the island of Ireland, and 50% of the Republic, is underlain by limestone.
With very minor exceptions, Irish limestones belong to two periods of geological history, one known as the Carboniferous (around 300–340 million years ago) and the other known as the Cretaceous (70–120 million years ago). The Carboniferous limestones are normally hard and grey to black in colour, and are found in almost every part of Ireland (every county except Antrim and Wicklow); the Cretaceous limestone (chalk) is somewhat softer and normally white in colour, and is found only in Ulster (Counties Antrim, Armagh, Down, Derry and Tyrone).
Geological map showing extent of limestones and chalk (Morgan Burke)
Limestones are rocks which are composed predominantly or entirely of calcite (calcium carbonate or dolomite (calcium/magnesium carbonate). Some limestones are almost pure carbonate, others contain substantial proportions of other material – most commonly sand, clay (mud or shale) and chert (very fine grained silica). The non-carbonate material may be distributed throughout the rock, may occur as small nodules (especially of chert) or may be concentrated in distinct beds (most commonly, beds of shale) inter-bedded with the limestone. The limestone may also contain small amounts of metallic minerals such as pyrite, marcasite, or galena, and in some places these may be concentrated in distinct veins or masses; where these mineral deposits are sufficiently large and concentrated to be exploited economically, they are termed orebodies, as found at Tara Mines (Navan), Galmoy (Co. Kilkenny) and Lisheen (Co. Tipperary).
Limestones can be formed in several different ways and in different geological situations, usually in the sea. They may be deposited in deep water far from land, in shallow water near the shore, or somewhere in between. Many limestones are predominantly composed of the calcareous shells or skeletons of marine organisms, but others are formed chemically by precipitation of carbonate from shallow waters. Some form in extensive horizontal layers (beds) which may be as thin as a few millimetres or as thick as several metres. Others form as massive unbedded banks or mounds of fine-grained calcareous debris (mud mounds), which can be many metres thick in the centre, thinning out towards the edges.
Dolomitic limestones (or dolomites) are rocks which have undergone chemical changes resulting in the replacement of some of the calcium by magnesium. Magnesium carbonate (the mineral dolomite) has a different crystal structure to calcium carbonate and this creates additional void space in the rock which can enhance the development of permeability and, in some cases, karstification.
The nature of the limestone strongly influences its susceptibility to karstification. Purer limestones are more susceptible than impure (shaly) limestones. Another strong influence is the geological structure: folding of the limestone causes fracturing and the formation of a network of fissures along which water can penetrate and begin to dissolve the rock. In general, pure limestones tend to be brittle, allowing extensive open fractures, while impure limestones tend to deform more readily, sealing up the fractures and impeding water movement. The degree of karstification is significantly reduced where there are inter-bedded shale layers which restrict water movement and where very strong deformation causes re-sealing of fractures with crystalline calcite.
Why are karst limestones important?
Karst limestones are important sources of water
Most of the largest springs in Ireland emerge from karst. Karst springs, both large and small, are ready sources of drinking water in areas where there are often no other alternatives due to the absence of adequate surface water courses. Numerous springs are venerated as holy wells, testifying to their significance for many hundreds of years.
Public supply well in a karst area at Turlough, Co. Clare (Donal Daly)
In recent years, many new water sources have been constructed by drilling deep boreholes for both public and industrial water supplies. Some examples include the public supply wells at Cloyne and Mitchelstown in Co. Cork; the wells supplying Dungarvan and Ardmore in Co. Waterford; and those in Athenry and Kinvara in Co. Galway.
Karst limestone is often encountered in civil engineering projects
Due to its particular characteristics, including an irregular bedrock surface, the presence of large voids and rapid underground drainage, karst limestone presents special problems for engineering projects such as roads, bridges, tunnels, sewerage pipelines and mining. Careful preparatory investigations are therefore required with special design measures and provision for unforeseen problems.
Karst regions are very important for heritage/tourism
The distinctive scenery of karst regions, especially the upland karsts, is a major attraction for tourists in all parts of the world. Much of the attraction of the Mediterranean tourist regions, for instance, depends on the contrasts between the white limestone cliffs and mountain slopes, and the deep blue sea. In Ireland, the upland karst regions such as the Burren, the Aran Islands, Ben Bulben, and the Cuilcagh mountains, have particularly attractive scenery. Elsewhere, lowland karst areas also have a distinctive and beautiful landscape, including for example such dramatic features as the Rock of Cashel, the shore of Lough Leane, Killarney or the turloughs in the Gort-Kinvara area.
The caves that are open to the public, such as Aillwee Cave (Clare), Crag Cave (Kerry), Marble Arch (Fermanagh), Dunmore Cave (Kilkenny) and Mitchelstown Cave (Tipperary), attract many to the magical underground scenery.
Many people take an active interest in exploring cave systems (caving or potholing). Speleology (the study of caves) is an important leisure pursuit, often combined with other interests such as botany, zoology, geology and photography.
|Limestone pavement and a Bronze Age wedge shaped tomb at Eanty Mor in the central Burren (David Drew)
Karst regions contain sites of archaeological importance
|Caves have always been used for shelter by animals and many cave systems house a distinctive and specially-adapted fauna. Humans have used caves for habitation from the earliest times and many important archaeological collections have been discovered in caves. Excavations in cave sediments have provided vital evidence of Ireland’s pre-history.
|Kesh caves - ancient caves abandoned by the water that formed them. They are now rich archaeological sites (Donal Daly)
In many areas, prominent hills of karst limestone have been used as sites for some of Ireland’s best-known archaeological sites, such as Dun Aengus on Inishmore, Dunamase Castle near Portlaoise, and for churches such as the Rock of Cashel for example.
Karst environments have a distinctive ecology
Karst areas often possess a distinctive ecology such as in the turloughs of Co. Galway and other limestone wetlands, and the distinctive flora of the Burren. These unique ecological environments are under threat from modern developments such as drainage, tourism, leisure activities, farming, industrial and urban development.