The area extending from the River Fergus (Co. Clare) in the south, north into south Mayo, east to the River Suck and west to the large Loughs of Corrib and Mask is mainly less than 70 m above sea level, with few hills to interrupt the typical extensive plains. Unlike those parts of the central lowland further east, this area does not have a thick covering of glacial deposits and the limestone bedrock is often exposed at the surface. Many karst features are therefore apparent, although most of them are not obvious to the casual visitor.
A large cavern (Pollaloughabo) on the lowlands between Gort
and Kinvara. A deep pool in the cave has been explored by
cave divers who have mapped an enormous water-filled
passage extending for more than 1km both upstream and
downstream (David Drew)
There are several major rivers draining the area (the Rivers Clare, Robe, Abbert and Dunkellin, for example) but for most of their courses these rivers flow in artificial channels constructed in the nineteenth century to help drain the land. Before that, nearly all drainage was subterranean. In the south of the area, and especially between Gort and Kinvara, drainage was never undertaken, and here the rivers disappear underground, emerge at the surface from springs, and sink underground once more in a very complex pattern. Most of the cave passages are completely water filled and explorable only by cave divers. It is known that a vast flooded cave passage up to 25 m in diameter carries the drainage from much of this area to large springs near Dun Guaire castle on the shores of Galway Bay at Kinvara. Inland from Kinvara, many collapsed hollows give access to short sections of this underground river.
The cave of Pollduagh, south of Gort, through which
the Gort River flows from the Punchbowl sink (David Drew)
In south Mayo, 10 km east south-east of Westport, the large Aille River descends from the Partry Mountains and vanishes underground at a spectacular sink. Speleologists have mapped an extensive network of passages, often containing deep water, between the sink and the reappearance of the river at Bellaburke, 3 km distant.
Map of the passages of Ballyglunin Cave in Co. Galway. The passages extend at very shallow depths beneath the River Abbert with little leakage occurring into the cave (David Drew)
The River Abbert, a tributary to the River Clare, looks like a typical, sluggish lowland river. However, when the river channel was being enlarged by blasting in 1955, the river suddenly disappeared down a hole in its bed. Flow was only restored when the newly opened swallow holes were blocked. Later, an extensive cave system was discovered at a depth of only a few metres below the surface and extending beneath the bed of the Abbert River. River water leaks into the cave at only a few points; the bed of the channel presumably being clogged with glacial clays.
Cross-section through a turlough showing the links between underground conduits and the turlough, and also the distinctive turlough vegetation. In the summer months the turlough is dry but in winter months, water may fill the turlough to the height of the dashed blue line (adapted from Catherine Coxon)
Seasonal lakes, called turloughs, are a partly karstic feature. More than 100 of them have been recorded, of which the great majority are in the western lowlands. Turloughs fill and empty, often unpredictably from openings on their margins. Commonly an opening can function both as a swallow hole and as a spring (estavelle), depending on the season. Turloughs have a very distinctive ecology (see diagram) due to the periodic flooding with lime-rich waters. Many turloughs have been drained in order to lessen flooding, including the largest in Ireland, the 250 ha Rahasane Turlough near Kilcolgan, Co. Galway. Only in south Co. Galway and around Ballinrobe in Mayo are there significant numbers of 'natural' turloughs.
Caherglassaun Lake, 5km inland from the coast at Kinvara (David Drew)
Caherglassaun is a small turlough, located 5 km from the sea, between Gort and Kinvara. Unlike other turloughs it never properly dries out and in summer it fills and almost empties twice daily in response to high and low tide at the coast. The rise and fall is lagged between 3 and 4 hours behind the high and low tides at Kinvara on Galway Bay.
The Lough Mask-Lough Corrib isthmus
One of the most remarkable karst landscapes in Ireland is found on the narrow neck of land which separates Lough Mask from Lough Corrib. The waters from Lough Mask sink underground at numerous points on the southern shore and flow at shallow depth via many fissures and caves to emerge from the various springs in Cong village, some 5 km to the south. The ground between the lakes resembles Gruyere cheese, being riddled with large rifts, caves and chambers hollowed out by the underground waters. The greatest concentration of swallow holes is to the east of the entrance to the Cong Canal: here the rocks form a chaotic landscape of hollows and tilted blocks where collapses have taken place into the caves beneath.
The Cong Canal and underground flow routes for water between Loughs Mask and Corrib (David Drew)
|The Cong Canal|
A canal linking Lough Mask with Lough Corrib was excavated in the 1840's as a Famine relief project. The canal was intended to lower winter water levels in the higher lake, Lough Mask, and to provide a navigation channel linking the lakes. The second objective was not realised as the canal encountered highly fissured karstic limestones which swallow up the canal waters during the summer months, leaving it completely dry.