The southern valley karst in Counties Cork, Kerry, Tipperary and Waterford

The karst in the southern counties of Ireland differs in a number of ways from that further north. The limestones in the south have been quite intensely folded: the further south, the more intense is the folding. This resulted from a mountain-building episode (the Variscan Orogeny), some 280 million years ago at the end of Carboniferous times, which created an extensive system of faults and fractures in the rocks. These were later opened up by solution to form a karst network. The work of erosion on the folded rocks has resulted in a distinctive landscape of sandstone ridges (including the mountain ranges of the Galtees and the Knockmealdowns) separated by limestone valleys drained by large rivers such as the Suir, Lee, and Munster Blackwater.

The limestone valleys themselves show a distinctive topography: the centre of the valley, underlain by the main limestone formations, tends to be elevated a little above the land on either side. The lower lying areas occur where streams or rivers have more deeply eroded the softer shales and shaly limestones immediately overling the older Devonian sandstones. The limestones are often largely covered and obscured by Quaternary boulder clays and gravels but can often be seen as small hills bounded by steep limestone crags. These areas expose the main karst features - caves, swallow holes and springs. Although large areas of bare limestone are uncommon, there are many small outcrops and the subsoil cover is often very thin.

Within the limestones, several different units have been mapped. The most widespread is the Waulsortian Limestone, comprising the coalescence of massive calcareous mud-mounds. Other limestone formations are more recognisably bedded. Investigations so far have not allowed detailed distinctions between the hydrogeological properties of the various clean limestone units to be made. In general, the Waulsortian Limestone is a better aquifer than the more shaly, bedded limestones, and is therefore considered to be more karstified.

An important factor in the development of karst in the southern counties was the relative lowering of sea level, by over 60 m, which is believed to have occurred during the Tertiary and Quaternary eras of geological time. When the sea level was so much lower, the rivers were able to cut deeply into their beds, and karst solution could penetrate well below the present-day water table creating a high porosity and permeability. When the sea level rose to its current level, the karst voids filled with water, giving rise to highly permeable limestone aquifers which are important water resources today.

Eagle's Wing, Mitchelstown Cave (Terence P. Dunne)

Examination of limestone exposures in many places, especially at the coast (e.g. at Fenit, Co. Kerry) or on lake shores (e.g. Killarney) shows numerous open joints and surface karstification. Evidence of deeper karstification along these joints can be found in cave surveys, which often show main passages or galleries about 1-6 m apart. Cloyne Cave in Co. Cork, is one of the best examples of a grid network of passages. The southern counties include numerous important karst features: swallow holes which take in surface runoff from the adjacent hills (e.g. Thonoge sink, near Ardfinnan, Co. Tipperary and Water Rock sink, near Middleton, Co. Cork); large karstic springs (e.g. Dower spring, near Castlemartyr, Co. Cork and Tobermaing, Castleisland, Co. Kerry); and large caves, including show caves such as Crag Cave, near Castleisland and Mitchelstown Cave. Another well-known cave is Castlepook Cave, near Doneraile, which contained abundant and important archaeological remains and is consequently sometimes referred to as 'Mammoth Cave'. The large karst springs, many of which are important regional or local water supply sources, and the relatively low lying aspect of the limestone valleys, ensure that the rivers of these areas maintain a reasonably constant flow through the summer.