The Distribution of Irish Rocks

The geological map of Ireland displays a wide variety of rock types which have originated at different periods of geological time. The oldest rocks are metamorphic gneisses which are to be found on Inishtrahull, several miles off Malin Head in Co. Donegal, and elsewhere in the north-west. They originally formed as igneous rocks 1750-1780 million years ago.

Other very old metamorphic rocks also occur, notably in west Galway and Co. Donegal, and in the Kilmore Quay and Rosslare Harbour areas of south Wexford. Included in the metamorphic category are the slates that have given rise to the great slate quarry at Valentia Island, Co. Kerry, and the quartzites that are found, among other places, in Counties Donegal and Wicklow. Because of their distinctive shattering during weathering, quartzites have produced some of Ireland's most prominent peaks, most notably Errigal

Errigal Mouontain, Donegal

Errigal is the highest mountain in the Derryveagh Mountains area and the 76th highest in Ireland.

Courtesy of Simon Stewart

 , Co. Donegal (752 metres), and the Great Sugar Loaf

Sugar Loaf

The quartzite peak of the Great Sugar Loaf, Co. Wiclow

courtesy of the Geological Survey of Ireland

 , Co. Wicklow (501 metres).

Extrusive igneous rocks are extensive in the north-east where the basaltic lavas have given rise to the Antrim-Derry plateau. Intrusive igneous rocks are represented by granites of several ages, in Counties Donegal, Armagh, Down, Wicklow and Galway. In Co. Wicklow the granites formed into a large dome structure known as a batholith. Remnants of the rocks which once overlaid the granite are evident in metamorphic schists at high altitudes and in a small band that borders the granite at lower levels. Now the granite has weathered into rounded hills. Some of these hills, for example Three Rock Mountain, Co. Dublin, Djouce, Co. Wicklow, Bloody Foreland, Co. Donegal, are surmounted by tors - distinctive blocks of rock that have weathered less rapidly than their surrounds.
Sedimentary rocks are widespread. They include the Old Red Sandstones from the Devonian period. These are particularly extensive in the 'ridge and valley' landscape that runs across much of south-west Ireland, in parts of Counties Tyrone and Fermanagh, and as 'inliers' of older rock protruding as small hill or mountain groups, like Slieve Bloom and Slieve Aughty, in southern parts of the central lowlands. Other sedimentary rocks include the shales, grits and occasional coal measures of the Upper Carboniferous period. Although long eroded from most parts of Ireland, they remain capping and protecting earlier rocks (usually Carboniferous limestone), on the plateau landscapes of Castlecomer and Slieveardagh, on the Kerry-Limerick-Cork borders, and in some parts of the northwest (e.g. at Benbulben).

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