The north-western plateau karsts
Upland karsts and associated cave systems are well developed in Counties Sligo, Leitrim, Cavan and Fermanagh. The area is largely a dissected plateau with caps of shale and sandstone on some of the mountains including Truskmore summit, Dartry Hills, Dough Mountain, Tullybrack, Belmore Mountain, Cuilcagh, Slieve Anierin, Carrane Hill and Knocknarea, and many cliff-edged, glacially-deepened valleys. Most of the karst and caves are developed around the edges of these mountains and on the sides of the associated valleys. A good example is Reyfad Pot on the north-east side of Tullybrack. Streams starting on the impervious shales and sandstones sink into swallow holes when they reach the limestone and descend into impressive vertical potholes. They then pass through the caves of the deepest pothole in Ireland (over 180 m deep), before re-emerging at Carrickbeg spring at the foot of the mountain. Mountains which have lost their capping of impermeable rocks include much of the Truskmore uplands, the Castlegal-Leean Mountains, O'Donnell's Rock, Keshcorran and the Bricklieve Mountains. In the absence of a shale cap to concentrate the drainage, the karst has developed a different character: there are no typical swallow holes and only rare access to any cave systems. Rainwater sinks underground, either through a diffuse system of enlarged joints such as on Leean Mountain, or by being drawn into fossil potholes formed on major joints and faults such as in the Largy Rifts at the east end of the Truskmore. Here the surface is characterised by enclosed depressions, dry valleys and limestone pavements.

Karst of the Northwest region: 1a, Knocknarea; 1b, Castlegal; 1c, Leean; 1d, Lough Gill; 2, Truskmore Massif; 2a, King's Mountain; 2b, Gleniff; 2c, Barytes Mine; 2d, Truskmore summit; 2e, Largy; 2f, Teampol Shetric; 2g, Barracashlaun; 2h, Outliers; 3, Dartry Hills; 4a, SaddleHill-Dough Mountain; 4b, O'Donnell's Rock; 4c, Dromahaire; 5a, Geevagh; 5b, Kilmactranny-Corran Hill; 6a, Lecarrow; 6b, Bricklieve Mountains; 6c, Keshcorran; 7a, Slieve Anierin; 7b, Skeemore Hill; 8, Aghaboy; 9, East Cuilcagh; 10, Tullyhona; 11, Prod's Pot-Cascades; 12, Marble Arch; 13, Hanging Rock-True; 14, Shannon Cave; 15, Pollnagossan; 16, Belmore; 17, Boho; 18, Reyfad; 19, Noon's Hole; 20, Knockmore

The variations in limestone types affect the development of the karst landforms and cave passages. In much of the area, caves are formed in cherty, bedded limestones on major joints or faults. The initial sections of the caves are often narrow pothole rifts whose walls are characterised by brittle chert ledges. If the base of the potholes are not choked with debris, it can be seen that vertical step profiles to the cave systems develop until a level close to that of the outlet springs are reached, as in Polliska, Co. Sligo (see Figure below). The collapse of one of these ledges, which was being used as a handhold by Ida Tighe of Sligo in Teampal Shetric, Sligo, caused her to fall 27m down a pothole to her death in 1935, the first recorded sporting caving fatality in Ireland.

Influence of limestone type on cave profiles (Gareth Ll. Jones)

In some areas, and especially in Fermanagh, the pure Dartry limestone allows the development of caves such as the Marble Arch and Reyfad Systems. If the entrance leads to a pothole, this frequently descends in a single or complex shaft to the lowest level, producing an L-shaped profile to the cave system such as is seen in the Noons-Arch System. The passages reach impressive sizes in this stronger, more competent rock. High on the mountains at Gleniff, Diarmaid and Grainne's cave is located in this same limestone type. At Boho in Co. Fermanagh, the limestones host an unusual joint-controlled maze cave and at Knockmore (Fermanagh), Pollraftara cave formed along a major inclined fault which can be seen through much of its 3.1 km length.

The cliff at Ben Bulben marks the edge of the plateau karst (Dan Tietzsch-Tyler)

Many of the older larger cave systems were initiated before and during the glacial period. This is evident where glacial debris has been deposited within the cave system. A significant feature of the area is the glacial modification of karstic landforms, seen most dramatically in the glacial truncation of the old system of Diarmaid and Grainne's Cave in Gleniff. The topography has changed so much in this valley, that it is difficult to envisage the nature of the original cave system. The enlargement of the impressive frost-shattered entrance to the cave, suspended high in the cliffs, was caused by the cold climate at the end of the Ice Age.

Looking out from the vast entrance to Diarmaid and Grainne's
cave, Gleniff, Co. Sligo (Pam Fogg)