Geology in Derry
About 3 miles to the east of Derry (Self Catering, Derry, Ireland) is the small glen of Lincairn. This gorge is 100 ft. deep and about two-thirds of a mile long, and has a level bottom. This must have been an overflow channel begun at a time when the Faughan valley had 250 ft, of ice in its bed, thus obstructing the drainage in that direction. The curious steep-sided pools that are met with in the gravels and sands on the right bank of the Foyle Kettle-hole, two miles N.E. of Derry (Accommodation, Derry, Ireland) north of Derry, mark the places where lobes of ice became separated from the main body of the glacier. Sands and gravels were deposited over and around them, and afterwards when melting took place, these kettle holes, as they are called, were formed. At the close of the glacial period the land was higher than now, for we have, in addition to the geological evidence, that of the relict fauna and flora, for believing that a land connection with Norway remained after that with the Continent by way of France was broken. Then came a depression that allowed the sea to re-enter certain of the river valleys of the north. Carlingford Lough was thus invaded, and Belfast Lough extended farther to the west than now, and the estuarine clays on which the lower part of Belfast now stands were laid down. An old shore line can be traced from Belfast round the Derry and Derry (Holiday Apartments, Derry, Ireland) coasts, showing that at this time the sea stood some 25 ft. higher than it does to-day. Afterwards, an uplift occurred that brought the sea to its present position and exposed the stumps of ancient forest trees and the peat of formerly submerged bogs.
Old records and tradition point to the fact that Inishowen was, up till very recently, an island, separated from the mainland by a narrow strait that joined Loughs Foyle and Swilly along the line of the present Lough Swilly railway. Farther south, the low tract from Blanket Nook to Carrigans suggests the probability that the tract to the north was similarly cut off. Inch island retained its insular character till quite recently, when the shallow strait was reclaimed from the sea by artificial embankments; and the hill upon which the city of Londonderry was built was an island in the Foyle until, by the building of walls on the south-west and north, the sea was kept out of what is now the valley to the west of the old city.
South-west of Derry the drift is of considerable thickness, and the soils arc fertile owing to the variety of the materials from which they arc derived. About Omagh we have again a rich assortment of different rocks in the drifts-grit, limestone, basalt, and altered slate.
In Derry city a granite sett from Carrigart has been in use for some time. A diorite from Fincarn, north-east of Buncrana, is used for the roads near the city ; and for those farther south grits and basalts are employed.