Summary of site:
Irish deposits equivalent in age to the productive Coal Measures of Britain are few and until the 1990s it was believed that the Carboniferous rocks of the Coalisland Coalfield were the only representatives in Northern Ireland. The reappraisal of the geology of the Fintona Block placed a large area of rocks formerly thought to be Devonian in age (between 400 and 350 million years old) into the Carboniferous, close to the end of the period. The rocks of the Slievebane Group were dated from microscopic plant spores which indicated a stage called the Wesphalian B, the same age as the most productive Coal Measures of Britain, formed around 310 million years ago.
In strong contrast with the deltaic swamp sediments of the British Coal Measures, the rocks in Straduff Quarry are coarse cobble and pebble conglomerates of the Drumlish Conglomerate Formation. The particles are in direct contact with each other, sometimes with a matrix but often without. The rocks are virtually unbedded and the pebbles, cobbles and boulders are fragmented volcanic rocks, mostly lavas classified as trachybasalts or trachyandesites. Occasional thin layers of hard, lime-rich sandstone with siltstones and reddish-brown to purple shales divide the conglomerates at long intervals and show the beds to be almost vertical. In the quarry only about 70 m of the lower part of the formation are seen. The total estimated thickness of the entire formation is around 1,000 m.
The rocks were formed in near desert conditions with strong erosion of volcanic uplands providing the material that spilled in the form of huge debris fans into the lowland basin, propelled by periodic flash floods. Faulting defined the margins and depth of the basin early in its evolution but as time progressed activity declined, the uplands were degraded until finally only fine sediment reached the fans.
There are resemblances between these rocks and their age equivalents in north eastern Canada, and Rhode Island and Massachusetts in the United States. All were part of the continent of Laurentia at that time and just a few degrees south of the equator.
The Drumlish Conglomerate Formation is unique for rocks of this age in the British Isles and consequently the Straduff Quarry assumes considerable importance both nationally and internationally. It is an important research resource allowing intercontinental interpretation of events in Westphalian B times around 310 million years ago. The quarry should be closely monitored for the rest of its working life and assessed as an ASSI either now or immediately it ceases activity.