County Down Geology

Samuel Lewis

1837

The Mourne mountains, extending from Dundrum bay to Carlingford bay, form a well-defined group, of which Slieve Donard is the summit, being, according to the Ordnance survey, 2796 feet above the level of the sea, and visible, in clear weather, from the mountains near Dublin: granite is its prevailing constituent. To the north of these mountains, Slieve Croob, composed of sienite, and Slieve Anisky, of hornblende, both in Lower Iveagh, constitute an elevated tract dependent upon, though at some distance from, the main group. Hornblende and primitive greenstone are abundant on the skirts of the granitic district. Mica slate has been noticed only in one instance. Exterior chains of transition rocks advance far to the west and north of this primitive tract, extending westward across Monaghan into Cavan, and on the north-east to the southern cape of Belfast Lough, and the peninsula of Ardes. The primitive nucleus bears but a very small proportion, in surface, to these exterior chains, which are principally occupied by grauwacke and grauwacke slate.

In the Mourne Mountains and the adjoining districts an extensive formation of granite occurs, but without the varieties found in Wicklow, agreeing in character rather with the newer granite of the Wernerians: it constitutes nearly the whole mass of the Mourne mountains, whence it passes across Carlingford bay into the county of Louth. On the north-west of these mountains, where they slope gradually into the plain, the same rock reaches Rathfriland, a table land of inconsiderable elevation. Within the boundaries now assigned, the granite is spread over a surface of 324 square miles, comprehending the highest ground in the North of Ireland. Among the accidental ingredients of this formation are crystallised hornblende, chiefly abounding in the porphyritic variety, and small reddish garnets in the granular: both varieties occur mingled together on the top of Slieve Donard.

Water-worn pebbles, of porphyritic sienite, occasionally containing red crystals of feldspar and iron pyrites, are very frequent at the base of the Mourne mountains, between Rosstrevor and Newcastle: they have probably been derived from the disintegration of neighbouring masses of that rock, since, on the shore at Glassdrummin, a ledge of porphyritic sienite, evidently connected with the granitic mass of the adjoining mountain, projects into the sea. Greenstone slate rests against the acclivities of the Mourne mountains, but the strata never rise high, seldom exceeding 500 feet. Attempts have been made to quarry it for roofing, which it is thought would be successful if carried on with spirit. Feldspar porphyry occurs in the bed of the Finish, north-west of Slieve Croob, near Dromara, and in a decomposing state at Ballyroany, north-east of Rathfriland. Slieve Croob seems formed, on its north-east and south-east sides, of different varieties of sienite, some of them porphyritic and very beautiful: this rock crops out at intervals from Bakaderry to the top of Slieve Croob, occupying an elevation of about 900 feet.

Grauwacke and grauwacke slate constitute a great part of the baronies of Ardes, Castlereagh, and the two Iveaghs: it is worked for roofing at Ballyalwood, in the Ardes; and a variety of better quality still remains undisturbed at Cairn Garva, south-west of Conbigg Hill. Lead and copper ores have been found in this formation at Conbigg Hill, between Newtown-Ardes and Bangor, where a mine is now profitably and extensively worked. Two small limestone districts occur, one near Downpatrick on the south-west, and the other near Comber on the northwest, of Strangford Lough. The old red sandstone has been observed on the sides of Strangford Lough, particularly at Scrabo, which rises 483 feet above the lough, and is capped with greenstone about 150 feet thick; the remaining 330 feet are principally sandstone, which may be observed in the white quarry in distinct beds of very variable thickness, alternating with grauwacke. This formation has been bored to the depth of 500 feet on the eastern side of Strangford Lough, in the fruitless search for coal, which depth, added to the ascertained height above ground, gives from 800 to 900 feet as its thickness. The greatest length of this sandstone district is not more than seven miles; it appears to rest on grauwacke. Coal, in three seams, is found on the shores of Strangford, and two thin seams are found under the lands of Wilnmount, on the banks of the Lagan; there are also indications of coal in two places near Moira.

Chalk appears at Magheralin, near Moira, proceeding thence towards the White mountains near Lisburn, and forming a low table land. The quarries chiefly worked for freestone are those of Scrabo and Kilwarlin, near Moira, from the latter of which flags are raised of great size and of different colours, from a clear stone-colour to a brownish red. Slates are quarried on the Ardes shore, between Bangor and Ballywalter, and near Hillsborough, Anahilt, and Ballinahinch: though inferior to those imported from Wales in lightness and colour, they exceed them in hardness and durability. In the limestone quarries near Moira, the stone is found lying in horizontal strata intermixed with flints, in some places stratified, and in others in detached pieces of various forms and sizes: it is common to see three of these large flints, like rollers, a yard long and twelve inches each in diameter, standing perpendicularly over each other, and joined by a narrow neck of limestone, funnel-shaped, as if they had been poured when in a liquid state into a cavity made to receive them. Shells of various kinds are also found in this stone.

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