Although Precambian rocks dating back to more than 2000 million years have been identified in coastal sections at Rosslare and Kilmore Quay in Wexford, the most extensive older rock formations in the country are represented by those of the lower Palaeozoic and the youngest (60 million years ago) by those of the Tertiary [...]]]>

Although Precambian rocks dating back to more than 2000 million years have been identified in coastal sections at Rosslare and Kilmore Quay in Wexford, the most extensive older rock formations in the country are represented by those of the lower Palaeozoic and the youngest (60 million years ago) by those of the Tertiary Period. The Palaeozoic era consists of Cambrian, Ordovician and Silurian rocks (mainly shales). They are usually metamorphosed and trend in a south-westerly direction. The Cambrian and Ordovician rocks occur in south-east Wexford and east Wicklow. The Ordovician (Mainly black to gray slate or slaty argillite, argillite, black to dark-gray siltstone) also occur in Cavan, Monaghan, Armagh and in south-west Mayo. They occur also in the Appalachian Valley of the US. Furthermore, they form the core of a number of hills in and mountains of the south, notably the Commeragh, Galtees, Slievenamon, Keeper Hills, Aughtys and the Slieve Bloom. The most metamorphosed rocks of period, e.g. schists and gneiss and to a lesser extent quartzite, occur extensively in the Donegal-Derry-Tyrone region and also in west Mayo and west Galway. At the end of the Caledonian (Mountain Building) period, folding took place in an east-west direction and has given rise to southern Munster in particular, its typical east west trending hills and valley topography. The Old Red Sandstone occupies much of the area south of a line from Dingle Bay to Dungarvan and also the main component rock type of the Knockmealdowns, the Galtees, the Slievenamon Range, Slieve Beragh, Slieve Aughty and the Slieve Blooms. It also stretches from Pomeroy in Tyrone to Irvinstown in Fermanagh.

Towards the end of the Carboniferous (laying down of coal deposits) Period, about 300 million years ago, the sea levels dropped again and a large area of tropical swamp emerged, stretching around most of the equator. Trees falling onto the waterlogged forest floor formed peat deposits which over millions of years were compressed and heated by subsequent rock layers to form the coal deposits of the Castlecomer Coal-field. The extinction of so many plants and animals at the end of the Devonian had cleared the way for a wide variety of new organisms to evolve. Huge forests grew up in the swamps and these formed an ideal habitat for early amphibians.

The Carboniferous Period created more than 40% of our rock surface area. The entire Central Plain of Ireland is an extensive sheet of Carboniferous strata pierced by only a few inliers. Most of this is calcite-rich limestone which stretches from Fermoy in the south to Dungannon in the north and from Galway/Clare coast in the west to Dublin/Meath/Louth in the East. At the later stages of the Carboniferous Period, the sediments became more laden with organic debris and with silt clay particles. This resulted in the formation of the Upper Carboniferous shales, Coal Measures and Millstone grits. These occur most widely on the Castlecomer Plateau, the Slieveardagh hills and in North Kerry, West Limerick, West Clare and in Leitrim and Fermanagh. Because they are rich in clay and silt, they usually weather to form poorly-drained, difficult soils.