Northern Ireland - Lakelands

The Northern Ireland Lakelands occupy much of County Fermanagh, which shares a border with the Republic of Ireland. In addition to the many tranquil lakes there are sheer cliffs and mountains many composed of limestone, and possessing extensive cave systems including Marble Arch. The principal settlement is Enniskillen.

Sedimentary bedrock

The Lakelands are composed of older sedimentary bedrock compose of layers of limestone, sandstone and mudstone with thin coal beds, which formed between 360 to 300 million years ago in a variety of tropical environments such a warm shallow seas, swamps, rivers and deltas. This rock sequence can be divided into an upper sandstone-rich unit (Coal Measures and Millstone Grit) and a lower limestone-rich (Carboniferous Limestone) unit; their combined thickness is greater than 7 km.

The upper sandstones and mudstones contain coal seams and are potential sources for oil and gas. These rocks formed when vast quantities of sediment were transported by large river deltas. Occasionally the tops of these deltas were exposed, which allowed massive swampy forests to develop. After burial the vegetation from these forests was compressed to produce layers of coal. The limestones formed in shallow tropical seas. Being soluble, the limestones have been dissolved by rainwater percolating through them to form many caves, dry valleys and sinking streams. Landscapes where drainage is underground through caves in this way are called karst landscapes. Such features are especially well developed in the Cuilcagh Mountain area. The limestones form an important source of drinking water for public supply, with the water flowing through the caves and fissures in the rock. The limestones and their associated mudstones also include layers with potential as sources of shale gas.

Basement rocks

In only one place do basement rocks equivalent to those of the Down—Longford area come to the surface. However, evidence from deep boreholes across the region indicates that these rocks continue below the old sedimentary bedrock layers but over most of the area this is at a depth of several kilometres.