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Ballygawley Delta Complex - OverviewTyrone
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Summary Full report
Site Type: Inland exposure
Site Status:
District:
Grid Reference: H613725, H571679
Rocks
Rock Age: Quaternary
Interest
Other interest: No data, Deltaic sediments, Glacial deposit, Ice stagnation, deltas, meltwater channel, moraine, outwash

Summary of site:

Enclosed within the valleys of the Fintona Hills to the north and north west of Ballygawley, lying on top of the bedrock, are extensive sand and gravel deposits telling a complicated story of the period at the end of Irelandís final glaciation. This was a time when the ice sheets that formerly covered the entire area began to melt into separate lobes, blocking valleys and impounding lakes of meltwaters highly charged with sands and gravels. These deposits largely consisted of rock that had been pulverized by the flow of the massive ice sheets and then released as they melted in the warming climate.

The main interest is concentrated on three sites: 1, at Fallaghearn, where there are stranded deltas with sands and gravels; 2, at Lurganboy, where there is another delta with outwash deposits; and 3, the valley between Tiroony and Gortfin containing a series of sand and gravel ridges. Each site is individually described as: Ballygawley Delta Complex - Fallaghearn Deltas; Ballygawley Delta Complex - Lurganboy Delta and Outwash; and Ballygawley Delta complex - Tiroony Ridges. There is sufficient evidence from these sites to construct a related series of events forming a loose history from when glacial melting commenced to the time when the area became ice free. In the final phase of continental ice sheet activity during the late Devensian (the last major glaciation in Ireland), the regional flow across the Fintona Hills was from north to south. However, with sustained climatic warming the ice began to thin and eventually break up, creating two separate margins - one retreating north, the other south, of the hills on to the lower ground, perhaps leaving patches of ice stranded on the hills themselves. The lowland ice fields were initially substantial and blocked the major valleys, preventing the escape of meltwaters and thus creating lakes whose shores lay partly on the bedrock of the emerging hills and partly on the margins of the decaying ice mass. The lakes rapidly silted up with the newly released pulverized glacial rubbish, forming flat-topped, steep-fronted deltas in the deep water against the hills and sand and gravel beds in the shallow water. The highest delta tops surviving indicate at least one lake surface at around 233-235m above present sea level. Between Tiroony and Gortfin, along the line of the Glashagh Burn, the northerly retreating ice eventually released its temporary lakes and briefly stabilised nudging south matching its rate of melting. The gravels and sands released in this brief period formed linear mounds (moraines) along the ice front, now softened into the Tiroony Ridges. This period of ice field contraction and retreat appears to have been erratic, sometimes progressing quickly but also with intervals of stagnation. The southern ice margin that formerly blocked Toddís Leap (the upland col to the south of the Fallaghearn deltas) withdrew to the south while the Fallaghearn lake was still at a high level. Some lake water drained south through the col into the Clogher Valley, actually reversing the original northerly flow of meltwater responsible for the deltas themselves. In the final stages, following the drainage of the lakes, late meltwater crossing the deltas cut deep valleys into the unconsolidated sands and gravels and there is strong evidence of intense meltwater flow to the south west (along the Glenshagh Valley from the direction of Pomeroy towards Sixmilecross). Although the deltas have been extensively worked for sand and gravel and still represent a valuable resource, the visual impact of workings on the sites from the roads is not great and most of the delta surfaces and their outwash deposits remain intact and the meltwater incisions clearly evident. Efforts to preserve some of these features are still justifiable. The Tiroony Ridges and the meltwater channels cut into bedrock at Toddís Leap and Bernisk Glen are unscathed.


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