The Distribution of Irish Rocks

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