Drummahon
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Geology & landscape
Geological Feature: Rock Outcrop · Geological Period: Carboniferous · Rock Type: Conglomerate, Limestone, Quartzite, Sandstone, Schist, Shale
Description
Drummahon Carboniferous stratigraphy Drummahon is of geological importance because of the occurrence of coarse conglomerates at this locality, unique in the outcrop of the Carboniferous Skea Sandstone Member of which it is a part. Conglomerates such as those at Drummahon provide information on rocks exposed at the time of its formation, namely the rocks which were eroded to form the conglomerate. Fossiliferous limestone blocks show that strata in the upper part of the Ballyshannon Formation were exposed at this time and eroded to provide detritus for this Member. Rock is exposed at many places along a steep scarp feature at Drummahon. The most commonly occurring rock type, both in outcrop and as loose blocks, is conglomerate. Clast size is highly variable but rarely exceeds 0.1m. Clast composition is dominated by white vein quartz, but large blocks of pale green-grey quartzite are common with smaller amounts of grey phyllite and graphitic schist. Reworked pieces of shale and fine-grained sandstone also occur in small numbers. Orange-stained dolomitised strongly recrystallised limestone clasts are common and contain fossils, including nautiloids and a range of corals. The occurrence of these corals indicates a mid-to late Arundian age, and in the Kesh-Omagh area such fossils are restricted to the top of the Ballyshannon Limestone and the Bundoran Shale Formations. The orange-stained, gritty and coarse-grained nature of the matrix of this conglomerate is identical to the basal sandstone bed of the Skea Member. The composition and size of clasts in this conglomerate indicate a more proximal location to the source of the detritus which yielded both metamorphic rocks and newly exposed fossiliferous limestones, including possibly the Ederny Member, of the Ballyshannon Formation. Although orange-stained very fine conglomerates and very coarse-grained sandstones do occur within the northern part of the outcrop of the Skea Sandstone Member, coarse conglomerates are confined to Drummahon. https://www.daera-ni.gov.uk/publications/drummahon-assi