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Ballygawley Delta Complex - Lurganboy Delta and OutwashTyrone
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Summary Full report
Site Type: Inland exposure
Site Status:
Grid Reference: H590644, H586660, H605675
Rock Age: Quaternary
Other interest: No data, Deltaic sediments, Glacial deposit, deltas, outwash, delta

Summary of site:

Extensive, flat-topped, deltaic and outwash deposits dominate the landscape around Lurganboy, extending from the limits of the slightly higher deltas around Fallaghearn (see Ballygawley Delta Complex - Fallaghearn Deltas) across Foremass to the dramatic cleft of Bernisk Glen.

The recently opened sand and gravel pit above Lurganboy shows a section through the deposits and reveals something of the history of the area during the closing stages of the last major glaciation (the Midlandian) around 10,000 years ago, when the ice fields broke up, retreated and finally vanished. The sediments in the gravel pit are coarse and unstratified near the base, becoming finer towards the top where sands predominate and bedding planes become more defined. Also towards the top there are channel fillings where streams pouring off the higher ground excavated courses which later silted up. Channels cut across earlier channel fills, causing abrupt terminations of beds, and in some places ripple marked bedding planes can be seen. A second section shows stacked beds up to 1m thick, with coarse grit and pebbles near the base, fining upwards into thinning beds of coarse sand containing channels filled with fine sand. The inclination of the sands suggests a flow towards the west and north west. The story appears to be of a glacial lake fed by meltwater streams from the uplands to the south east that washed massive volumes of freshly released, glacially pulverized, rock fragments into the standing water. Eventually finer material settled on top until delta platforms of pebbles, grit and sand were established at the water surface. The upper surfaces were then carved into channels by distributaries crossing to deeper water. The sediment source for the deltas appears to have been the upper Owenbrack valley. As at Fallaghearn, the lake probably drained over a very short period (hours or days) as the ice or sediment dams containing it were breached, stranding the deltas and outwash deposits. Meltwaters continued to flow over the surface of the stranded deltas, carving channels but, unlike Fallaghearn, here they are wide and were filled by alluvium in later, warmer times. The only threat to the site is from sand and gravel workings but, if carefully managed, much of the evidence implicit in the sediments and their setting can be preserved for future study.

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