Traditionally known as the Upper Limestone, the Dartry Limestone is a significant landscape-forming rock unit dating from the Asbian substage. It is dark grey and argillaceous in nature and includes some fossils and chert nodules or beds. The type area is in the Benbulben Range and Dartry Mountains of County Sligo. Up to 350m thick, it is subdivided with several distinct ‘rock units' or ‘members' recognised. In stratigraphic order, these are: