Nephin Mountain RangeNorth Mayo is formed partly of rock from the Precambrian period, laid down over 600 million years ago. The Dalradian group from this period, which underlies the National Park, were laid down in a marine environment of current-swept shallows with sandy shoals, calcareous lagoons and deeps with muddy floors. The western side of the Park is underlain by schist and gneiss rocks, hardened and crystallised by burial and folding. Quartzite is dominant to the east and southern ends of the Park.

Glacial activity over the past 2.5 million years has created some of the most scenic features of the Park. These include the many corrie lakes such as Corryloughaphuill Lough. Glacial boulder clay, found at the southern edge of the Nephin beg mountain range, is further evidence of glacial activity.