The Structural Relations of the Mourne Granites, Northern Ireland

Abstract

The Mourne Mountains rise abruptly from sea-level south of Newcastle, Co. Down, on a front of 5 miles, and extend westwards as a conspicuous range of peaks and ridges for some 13 miles to the neighbourhood of Rostrevor. They are formed mainly of granite, with foothills to north, west, and south-west composed of contact-hardened Silurian shales. To the south-east a gently sloping plain lies between the granite hills and the Irish Sea. During field-work in 1925 it was found that the massif included more than one granite intrusion. At least four distinct intrusions, composed of different varieties of granite, are now known. Three of these occur to the east, the fourth to the west. Two of the Eastern Granites are in contact with the Western Granite midway between Newcastle and Rostrevor, but only along a length of about 2 miles. The massif in plan is thus of hour-glass shape (see fig. 1). Only part of the top of the Western Granite is exposed, for it extends on all sides below a slightly domed roof of Silurian shales. In the case of the Eastern Granites, roofs of shales or of older granite are well seen, and, in addition, erosion has extended deep enough to disclose parts of bounding walls to all these three intrusions. In the eastern half of the Mournes, many peaks are upwards of 2000 feet in height, the highest being Slieve Donard at 2796 feet. On the west the hills are much lower, excepting the two adjoining centrally