The higher mountains are of quartzite which weathers into bare, cone-shaped peaks such as Errigal (752 m) in Donegal, Croagh Patrick (765m) in Mayo and the Twelve Bens in Galway. Structures of similar age are responsible for the Wicklow and Blackstairs mountains which extend south-westwards from Dublin Bay for a distance of more than 100 km. In these, long-continued denudation of a great anticlinal structure has exposed a granite core which now forms rounded peat-covered uplands, the crests being notched in places by glacial cirques. The mountains are penetrated by deep glacially modified valleys of which the best known is Glendalough in County Wicklow.
The younger structures (Armorican) extend from central Europe through Brittany to southern Ireland, where they reappear as a series of east-west anticlinal sandstone ridges separated by limestone or shale-floored valleys. The hills rise in height westwards culminating in Carrantouhill (1041 m) in the Magillycuddy Reeks, the highest mountain in the country. The famous Upper Lake of Killarney nestles in the eastern slopes of this range. The valleys separating the western extension of these mountains have been flooded by the sea, giving rise to a number of long deep inlets. In north-eastern Ireland basaltic lavas spread widely over the existing rocks in Eocene times and now form the bleak plateau of east Antrim. Westwards
the basalt is downwarped and the resultant drift-covered lowland is occupied in part by Lough Neagh, the largest lake in Ireland. The heart of the country is a limestone- floored lowland bounded on the south by the Armorican ridges and on the north and west by the Caledonian mountains. This lowland is open to the Irish Sea for a distance of 90 km between the Wicklow Mountains and the Carlingford peninsula, giving easy access to the country from the east. It also extends westwards to reach the Atlantic Ocean along the Shannon Estuary, in Galway Bay, in Clew Bay and again in Donegal Bay.
