"The denudation chronology of the mountainous district of south-west Mayo and north-west Galway is closely linked with drainage superimposition from a deformed Carboniferous cover. Small outcrops of basal Visean sandstones and conglomerates, lying on bevelled summits and wide bevelled tablelands demonstrate unequivocally that the west Connaught summit level corresponds to the base of the Carboniferous. Uplift, to produce a pre- glacial divide, is believed to have occurred in mid-Tertiary times. Upon the uplifted, faulted, and warped Carboniferous cover a consequent drainage was developed. Superimposition on to the Caledonian basement followed and during the Pleistocene considerable glacial modifications of the pre-glacial drainage pattern took place."