The limestones of the east midlands, stretching from the River Shannon and its lakes to the Irish Sea, and from Monaghan to Kilkenny, would not generally be associated with karstification. These limestones are mantled by a variable thickness of glacial and recent sediments - sands and gravels, tills (boulder clays), peat and alluvium (river silts and clays). The small-scale relief is provided by eskers, drumlins, raised bogs and river flood plains, which leaves the area resembling a plain of glacial deposition rather than a karst landscape. Isolated hills that rise up to 60 m above the surrounding lowlands consist of limestones peeping through the glacial deposits. Sandstones and shales provide the higher topographic features, such as Slieve Bloom, the Devil's Bit mountains and the Castlecomer Plateau. The surface water features - rivers, streams and lakes - are typical fluvial systems. So, unlike the Burren, the Fermanagh area or the western lowlands, there is no readily appreciable influence of karstification. However, closer examination shows many typical karst features are present, at least in some areas, which gives rise to the questions: what is the evidence of karstification? what has influenced karstification? and how karstified is the area relative to other areas?
Evidence of karstification
The main evidence of karstification comes from the presence of caves in many of the isolated hills, swallow holes in areas such as Monaghan and around the Castlecomer Plateau, infilled solution pipes and cavities in wells.
The steep, rocky, isolated limestone hills in Counties Westmeath, Offaly, Laois and Kilkenny may be residual karst hillocks (called tower karst), similar to those found in many lowland karsts in, for instance, the Mediterranean, south China and many tropical areas. The presence of what are obviously fossil remnants of formerly active caves such as Poll na gCat in Westmeath, and Mount Briscoe Cave in Offaly, support this view.
In the Carrickmacross area in Co. Monaghan, sinking streams, resurging rivers, caves and collapse features are present. Among the best known features are Finn McCool's Cave and Tiragarvan Cave. Further south, in the Drogheda area, cavities in boreholes and quarries provide the evidence for ancient karst.
In Counties Offaly, north Tipperary and Laois, some wells contain cavities; swallow holes are present but are not common; and solution pipes infilled with Tertiary clays occur at Ballymacadam, Hollymount and Ballygaddy.
In Kilkenny, there are numerous karst features such as large springs (in the Nuenna valley); swallow holes (south-west of Kilkenny in the Clonmoran/Shellumsrath area); underground drainage (in the Ballintaggart area, west of Callan); caves (Dunmore Cave), dry valleys and karst depressions (Nuenna valley).
Fore springs, Co. Westmeath (David Drew)
Lough Lene
The lakes of Co. Westmeath straddle the watershed between the River Inny (River Shannon) and River Deel (River Boyne) catchments. It was remarked by Piers in 1682 that Lough Lene drains via a surface channel eastwards to the River Deel and subterraneously via sinkholes to springs in the settlement at Fore to the north, and hence flows both to the Atlantic and to the Irish Sea. A recent hydrological study demonstrated that water from the swallow holes on the shores of the lake does indeed flow underground to the southerly spring at Fore with a flow rate of 80 m per hour. |
Dunmore Cave
There are few explorable caves in the east midlands but Dunmore cave, north of Kilkenny city, is one of the exceptions. It appears to be one of the oldest caves in Ireland and is a remnant of a much larger cave system, thereby providing evidence of a mature palaeokarstic landscape in the area. It consists of a number of large caverns linked by a fossil stream cave. These caverns contain many fine examples of stalagmites and stalactites. The cave is important archeologically and historically and there have been numerous finds of human and animal skeletal remains, along with other interesting artefacts. The discovery of coins of Viking age supports the account in the Annals of the Four Masters that the Vikings massacred many people in the cave in 929 AD. |
Dunmore Cave, Co. Kilkenny (Terence P. Dunne)
What factors influence karstification?
Three factors in particular seem to have influenced the degree of karstification in the east midlands:
- the lithology (type and content) of the limestones;
- the presence of low permeability sandstones and shales nearby;
- the geological history.
While karst features are present in all limestone types, they are far more common in the purer limestones than in muddy limestones. Muddy limestones underlie a high proportion of the east midlands (perhaps 70%), in particular in east Co. Offaly, Co. Dublin and Co. Meath. Karstification does not seem to be significant in areas underlain by these rocks. Where pure limestones are located close to areas underlain by sandstones and shales (for instance, at Carrickmacross and around the Castlecomer Plateau), water with a high dissolving capacity flowing on to the limestones has increased the degree of karstification.
The geological history of the area during Tertiary (from 65 to 1.6 million years ago) and Quaternary times (1.6 million years ago to today), while poorly understood, helps explain the present day situation. It is probable that most of the solution of the limestones occurred during the Tertiary, forming a mature karst landscape in the process. During the later part of the Tertiary, much of this landscape was dissolved and eroded, leaving residual features such as the isolated hills, caves and swallow holes.
How karstified is the area relative to other areas?
'Less karstified' is the best answer at present. A definitive answer is not possible as few studies have been conducted and the mantling of the limestone by the glacial deposits may be hiding more karstification than we are aware of. However all the evidence, particularly from boreholes, suggests that groundwater flow in fissures is more important than flow in large conduits. The predominance of muddy limestones in many areas has reduced the degree of solution. Well yields are more predictable than, for instance, in the western lowlands.