the most expansive sections are the Lower Carboniferous limestones and sandstones of south Donegal, the Devonian granite of the Donegal batholith, which extends for about 75 km northeast to southwest from Ardara to Fanad Head, and quartzo-feldspathic Dalradian rocks from the Precambrian era, which cover much of the rest of the county. The geology of Donegal is very similar to that of County Mayo, and both counties are located within the Grampian Terrane. The oldest rocks in Ireland are a granitic gneiss found on the island of Inishtrahull, located ca. north-east of Malin Head. They are 1.78 billion years old, making them Paleoproterozoic in age. The oldest rocks on mainland Donegal are a quartzo-feldspathic paragneiss found around Lough Derg, which have been dated to 1.713 billion years ago. Donegal is the most seismically active part of Ireland. The Leenan Fault is a large strike-slip fault that bisects the county from Lough Swilly to Donegal Bay, and dozens of tremors have been recorded along the fault since the late 19th century, although none have been larger than a magnitude 3 on the Richter scale. The county's landscape was carved out by glaciation at the end of the Pleistocene and the subsequent retreat during the early Holocene. Donegal contains one of Ireland's three glacial fjords (or fjards) at Lough Swilly, the others being Carlingford Lough and Killary Harbour. Lough Swilly is the county's largest inlet and forms the western boundary of the Inishowen Peninsula. The thick ice sheet that once covered the region carved out deep basins in the Donegal uplands and many tarns or corrie lakes developed in these depressions after the ice had melted, including Lough Maam below Slieve Snaght and Lough Feeane under Aghla More. Larger glacial lakes formed in the county's distinctive U-shaped valleys, such as Lough Beagh and Gartan Lough in Glenveagh, and Dunlewey Lough and Lough Nacung Upper in the Poisoned Glen. Valleys such as these were the last areas to retain glaciers as temperatures rose. As the ice sheet thinned, topography became the dominant force driving the direction of ice and meltwater flow. Erosion by glacial meltwater carved out large channels in southern Donegal which directed water and sediment to outwash fans in Donegal Bay. The area south of Donegal town, where the River Eske flows into the bay, is an example of one of these outwash areas. Sea levels in the area began to