Drumbally Hill

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Londonderry County

Habitats

Fast flowing river and stream

Geology & landscape

Geological Feature: Rock Outcrop · Mining / Industrial: Quarry · Geological Period: Carboniferous · Rock Type: Limestone, Shale

Description

Drumbally Hill Carboniferous stratigraphy The area is of special scientific interest because of its' important geology. The exposures are found on the south east side of Drumbally Hill. The site is what remains of a once more extensive quarry that exposes limestone typical of the Desertmartin Limestone Formation of the Armagh Group. The rocks are from the late Asbian stage of the Carboniferous and are some 320 million years old. Outcrop of the Desertmartin Limestone Formation is rare and the site also represents the northernmost occurrence of Asbian limestones in Northern Ireland. Up to 3m of limestones are exposed in the remaining quarry face, found near a stream in a wooded area. The strata are inclined at 15° to the southeast and exhibit undulose bedding ofvariable thickness and are shale-free. The limestones vary in colour from pale fawn-grey to medium grey but are usually stained pale purple-red on most surfaces, probably as a result of downward percolation of later Mesozoic groundwater. Medium to coarse grainstones dominate the sequence here, with thin layers of very sandy limestone and occasional quartz pebbles are scattered in the rock. A sparse, often fragmented macrofauna of crinoid ossicles and large productid brachiopods, in particular Linoprotonia hemisphaerica, is recorded from the limestones but corals are absent from this and most other quarry sections in the Desertmartin area. The rock would have originally been deposited as soft sediment in a warm, shallow marine environment. The presence of significant amounts of quartz sand and pebbles in the Limestone indicates a proximal, or near shore, depositional environment. Furthermore, this environment must have experienced turbulent or reasonably high energy conditions, which had the effect of winnowing out the lighter carbonate muds leaving the heavier quartz grains behind. https://www.daera-ni.gov.uk/publications/drumbally-hill-assi

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