Glaciology - Inishowen During the Ice Age
By Professor J.K. Charlesworth, D.Sc.
An excerpt taken from 'The Book of Inishowen' by Harry Percival Swan

During the Ice Age, the 1st chapter of the Earth's history, Inishowen, like the rest of north-west Europe, was completely buried beneath an ice-sheet. The ice, probably about 3,000 feet thick, swept over Inishowen from the Donegal Hills, where lay its source, and proceeded across hill and dale parallel roughly with the Foyle and Swilly. On the north of the Peninsula, it flowed off the coast and gave rise doubtless to icebergs in the Atlantic.

The ice, shod with boulders and other debris, scratched the rocks over which it passed and scored them in the direction of its own flow. It also by their means moulded the hills into the beautifully rounded forms which are everywhere apparent.

It also brought with it materials from the west, including the rounded granite boulders, derived from the Barnesmore Mountains in County Donegal, which careful search will find in all parts of the peninsula. It laid down the sands and gravels and clay which floor the valleys and lower parts of the country.

With the passing away of glacial conditions, the ice gradually diminished in thickness. It was no longer able to surmount the highest peaks, such as Slieve Snacht and Bulbin. These, therefore, emerged above the ice-surface and the once-continuous ice broke up into powerful glaciers which streamed from the south-west along Lough Swilly and Lough Foyle and along the Glentogher and Mintiagh valleys.

These glaciers shrank southwards. They withdrew from the Glentoher and Mintiagh valleys and only glaciers in Lough Foyle and Lough Swilly remained. The Swilly Glacier ponded a lake in the broad valley of the Oweneg River, north-east of Buncrana. The lake overflowed by the valley which begins just north of the Mintiaghs Lough and falls northward into the valley of the Clonmany River.

Subsequent to the release of Inishowen from its ice cover, the peninsula has risen about 20 feet. The raised sea-bench which marks the ancient shore and is no longer washed by the waves may now be seen as flat shelves, now in rock now as gravel beach, at intervals along the Lough Foyle shore, as south-east of Muff and flooring the depression between Culdaff and Carndonagh and extending west of the road from Carndonagh to Malin and north of the Carndonagh-Ballyliffin Road.

The great shingle accumulation, on the southern side of Malin Head, which contains pebbles and boulders of all kinds of rock brought by the ice from the south-west, is doubtless such a raised beach but probably also contains much storm material, hurled up in the past by great waves on this exposed coast.

The most magnificent example of this raised sea-floor, however, is that which forms Magilligan Strand, which stretches out into Lough Foyle opposite Greencastle. Its flatness provided an admirable base-line for the primary triangulation of the British Isles by the Ordnance Survey in 1827.

At this period when the sea covered the raised beaches the sea entered the depression which runs from Londonderry to Fahan from either end and the middle part which was probably not submerged was rendered marshy. Inishowen was at this time an Island.

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