County Fermanagh was named after the Irish Fir Manach meaning men of Manach. Most of the county are lowlands with numerous lakes and swamps, notably Upper and Lower Lough Erne and the River Erne. The county has three prominent upland areas, the West Fermanagh Scarplands (350 m asl), the Sliabh Beagh hills, and the Cuilcagh mountain range (665 m asl).
The oldest rocks are called red beds and were formed approximately 550 million years ago. But most of the county is covered by Carboniferous (354 to 298 Ma) shale and limestone with extensive cave systems such as the Shannon Cave, the Marble Arch Caves and the Caves of the Tullybrack and Belmore hills.