Deserts and Tropical Oceans

Desert conditions prevailed during the Devonian period, with Northern Ireland once again present as dry land. The erosion of a now vanished mountain chain and volcanoes yielded sediments which in turn produced new rocks - the 'pudding stone' conglomerates of Cushendun and red sandstone forming low ground on the north of the Clogher Valley.

Returning marine conditions during the Carboniferous resulted in the development of extensive limestone, some of which bear evidence of ancient reefs and tropical coral-fringed oceans. These can be seen in the Marlbank area of Fermanagh where they are testament to the great thickness which these reefs achieved. Some of the limestones in Fermanagh have subsequently been partially disolved by the action of rainwater. Cave systems have developed, with associated surface features including sinking streams and dry valleys. Some o the finest examples are to be found in the area of the Marble Arch show-caves.

Progressive shallowing of the ocean occurred later in the Carboniferous, until eventually great river delta's dominated the area. Great thicknesses of these sand and mud are preserved, forming much of the upland blocks of Slieve Beagh and west Fermanagh. Accumulated organic remains from these times, have also been preserved in places and form the small coalfields of Ballycastle and Coalisland.

While Northern Ireland remained dry land during the next geological period, the Triassic, a notable climatic change occurred. Now positioned approximately where Sudan is today, arid desert conditions prevailed. Occasional flood events deposited sand in shallow lakes, surrounded by dunes, producing the sandstone now seen at Scrabo and within the Lagan Valley.

A gradual return to shallow marine basins, susceptible to drying out under the intense evaporation, resulted in great thicknesses of red mudstone in places sandwiching massive beds of salt, the residue left as sea-water disappeared in the heat. While these soft rocks generally do not make a dramatic impact on landscape, exploitation, continuing today, has left evidence of a salt mining industry in the Carrickfergus area.