Carnmore
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Geology & landscape
Geological Feature: Rock Outcrop · Mining / Industrial: Quarry · Geological Period: Carboniferous · Rock Type: Dolomite, Sandstone
Description
Carnmore Carboniferous stratigraphy Carnmore is of importance because of its geology. The site is the stratotype for the Carnmore Sandstone Member, a new lithostratigraphical unit within the Carboniferous Meenymore Formation. The Carnmore area provides exposure of this exceptional thickness of sandstone within the Meeneymore Formation. The Carnmore Sandstone Member occurs within the Meenymore Formation. It is about 70m thick and its base occurs some 160m above the base of the Formation. Neither the upper nor lower contacts of the Carnmore Member are exposed. Its principal outcrop is confined to the prominent ridge that runs some 3km northeast-wards from Carnmore Lough to Carricknamaddy. While sandstones are one of the dominant lithologies of the Meenymore Formation they only rarely develop into a thick unit. This occurs within the Carnmore Member but also in the Derrygonnelly area where two sandstones are present, namely the Quarry and Glen Sandstone Members. However it is not possible to correlate either of these Members with the Carnmore Member. The Carnmore Sandstone consists of pale grey-fawn, very coarse-grained to medium grained litharenite with pebbles scattered throughout the matrix and concentrated in thin, discontinuous beds. Pebbles and coarse grains are composed almost exclusively of a variety of forms of quartz and also muscovite. Cement is usually absent with the exception of small amounts of clay minerals and iron oxide and, exceptionally, in a few layers, ferroan dolomite which imparts a bright orange stain to the rock upon weathering. The composition of the Carnmore Sandstone appears to indicate either a mature source area, probably composed of quartzose metamorphic rocks, or a derivation from a distant source area, thus allowing time for feldspars and softer lithologies to disintegrate and weather to clay minerals. The latter possibility is, however, not supported by the relative angularity of clasts, particularly the quartz grains, in the matrix. Crude ripple marks and medium and large scale trough-cross stratification, indicate that these sediments were deposited under the influence of water movement. https://www.daera-ni.gov.uk/publications/carnmore-assi