The Cabragh Wetlands are underlain by rocks of Lower Carboniferous age. They consist of Tournaisian limestones, dolomites and thin bands of shale. These limetsones were originally the remains of skeletons of sea creatures such as sea-lilies and other invertebrates. They formed in warm, shallow seas containing a rich fauna of sea lilies (up to 0.5m in height), corals, sea snails, trilobites (now extinct) and small fish. The remains of these can still be seen in the rocks as fossils. Lower carbonifereous shales contain fossil plant spoes which can be used to date and correlate rocks separated by large distances.
East of the Thurles area is the Slieve Ardagh Coalfield. The coal occurs in the rocks of the Upper Carboniferous ae and is indicative of the chance of environment within which these rocks formed. Warm, shallow seas were replaced by large deltas related to rivers flowing from the north. On the deltas grew swamp forests in which spiders, dragonflies, lizards and amphibians prolifereated. The plants included lycopods such as Lepidodendron whcih grew to 35m in height, horsetails which can still be seen in the wetlands today and a variety of ferns including tree ferns which grew up to 8m in height. The Carboniferous period also saw the flourishing of seed bearing plants. Rising sea levels drowned the swamps, burying the thick piles of vegetation under marine muds and sands. After burial the vegetation turned from peat to coal.
The most recent deposits to be found in the Cabragh Wetlands are overbank and flood plain deposits related to the River Suir.
