The Lettereeneen fault, a newly recognized structure, brings the Mweelrea and Maumtrasna groups of the Partry series (Caradocian-Llandeilian age) into contact. The stratigraphy of the Mweelrea group, of red bed facies, is followed from the presence of welded tuff horizons; no such markers exist in the Maumtrasna group which lies unconformably upon the former. Sedimentary structures of shallow water origin occur in each group. Three types of conglomerate recognized in the area are examined. The immature feldspathic sandstones increase in arkosity upwards. A proluvial or proluvio-marine environment of deposition is suggested, with debris derived from an eastward extension of the metamorphosed Dalradian rocks of the Connemara cordillera and foothills of sedimentary and volcanic rocks.