most northerly county of Ireland, in the historic province of Ulster. It is bounded on the west and north by the Atlantic Ocean, on the east by Lough (lake) Foyle and Northern Ireland, and on the south by Northern Ireland and the Irish county of Leitrim. The small village of Lifford is the county seat. The rugged coast is heavily indented, in the north, with major inlets being Lough Swilly and Lough Foyle, between which is the Inishowen Peninsula. The chief rivers are the Finn and Erne. The main mountain ranges are the Blue Stack, whose highest peak is Lavagh More (2,218 feet [676 metres]), and Derryveagh, which reaches 2,467 feet (752 metres) at Errigal. Evidence of extensive glaciation exists. The climate is temperate, with warm summers and mild, moist winters.
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