Geology of Inishowen
By J.J. Hartley, M.Sc.

With the exception of the Country around Londonderry, and the north-western border of Lough Foyle, the Inishowen Peninsula is built up from a very ancient series of rocks, known as the "Dalradian Series."

These rocks run across the peninsula in broad bands, each one extending from the north-east to the south-west, and sometimes as much as two or even three miles in width. They consist essentially of those types which are known as quartzites, schists and limestones, and they represent a series of marine sediments - sands, muds, and perhaps banks of shells - which have been compressed, however, and much altered in appearance through the long continued action of heat and pressure.

If we commence on the north-west coast and cross the county towards the south-east, we have first a quartzite band which is well exposed at Malin Head, This is succeeded by a belt of soft micaceous schist nearly three miles in width; and then we find another bank of quartzite extending from Culdaff through Carndonagh, and across the Crana river to Buncrana on Lough Swilly.

Since the quartzites are hard and tough rocks, they are strongly resistant to the action of the weather, and so usually form high ground. The Crana band, for example, has built up the central and highest mountain range of Inishowen with culminating peaks of Slieve Snaght, Slieve Main and Crocknamaddy.

Following the Crana quartzite, we find a thin bank of limestone - best seen around Culdaff and south of Carndonagh. This limestone is usually black rather than white, a somewhat unusual feature for this kind of rock.

The rest of the area is mainly occupied by a thick series of slates and grits. They also represent altered sedimentary material, but here the alteration to which they have been subjected has not been so intense as it was in the case of those rocks previously described.

At the top of the series there occurs a small area of greenish schistose beds. These are well developed around Greencastle, and probably account for the name of that particular locality.

Mixed up with these "Dalradian" rocks, which represent one of the earliest chapters in the long history of the earth, we find others which were squeezed into them in the form of hot liquids, and which afterwards cooled and solidified, to give rise to a different types of rocks. These include a small patch of granite found south of Dunaff Head, and hard dark green varieties known as "epidiorites" which are well exposed round the Mintiaghs and elsewhere. Both the King and Queen of the Mintiaghs are composed of this material.

For a good geological map, together with photographs and fuller details of the rocks, reference may be made to a paper by Dr.W. J. McCallien, to be found in the Proceedings of the Royal Irish Academy, Dublin, for 1935.

In conclusion, there follows a short note on the minerals of economic importance which are found in the peninsula. In central Inishowen, south of Carndonagh, an in the Glentogher valley, deposits of lead and silver ore occur. These were worked during the last century. A considerable amount of bog iron ore was also formerly exported both for gas purification and as a source of iron.

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