A massive intrusion of igneous rock that is usually granitic in composition and very deep-rooted. The batholith develops intrusively, i.e. its materials do not reach the surface as with lava, but they may be subsequently exposed as a result of erosion over millions of years.
The Leinster batholith that features the Wicklow mountains is the largest in Ireland or Britain. It dates from the Caledonian mountain building period of about 400 million years ago. Parts of it, notably the Wicklow mountains, have been subject to later uplift.
Other Irish batholiths with large bodies of granite rock which originated in Caledonian times are located in Donegal, Mayo-Sligo, Galway and in south Armagh-south Down. Much younger granites, dating from around 60-70 million years ago, are also to be found in the Mourne Mountains area of south Down.