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Foyle Valley ComplexLondonderry
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Summary Full report
Site Type: Inland exposure
Site Status:
District:
Grid Reference: C24134128, C23993858
Rocks
Rock Age: Quaternary
Rock Type: Gravel, Sand
Interest
Other interest: No data, Glacial deposit, Glacio-fluvial sediment, flat-topped delta, hummocky moraine, ice-marginal lake, moraine ridge

Summary of site:

This account describes the retreat of the ice sheet at the end of the last Irish glaciation (the Midlandian) from a roughly triangular area with corners at Castlederg and Newtonstewart in the south and New Buildings just south of Londonderry in the north. The area includes a network of valleys draining into the River Foyle, including the Glenmornan, the Burn Dennet, the Mourne, the Derg and the Strule. The Burngibbagh meltwater channel drains into the River Faughan which enters Lough Foyle directly.

Immediately south of New Buildings hummocky moraine ridges with a north-west/south-east orientation occur on the eastern bank of the Foyle. They lie across rock ridges with a north/south orientation carved into the Ballykelly Formation of Dalradian rocks beneath. To the east, flat topped deposits occur in what must have been the intake for the overflow of meltwater into Burngibbagh, the deeply incised valley feeding the lower Faughan. More north/south rock ridges can be seen north west of Bready with unexposed flat-topped deposits to the east. At Cloghcor, hummocky and kettle-holed moraine material forms a patch of high ground between the Foyle and Burn Dennet valleys, with a spread of sand and gravel to the north east sloping between 20m and 15m above sea level. A little to the north, east of Lough Neas, an abandoned sand and gravel working reveals a deposit of pebble- and cobble-filled channels, up to 5m thick with rippled sands. This is topped by 80cm of boulder and cobble gravel with crude stratification. An ice wedge (over 1m deep) cuts these deposits, indicating a period of intense cold after they were formed. From Grove Hill to Crockanroe, on both sides of the Glenmornan valley, there are flat-topped remnants of what was previously a deposit filling its entire bottom. Their tops reach a level of 160m above sea level. On the southern side of the valley near Murder Hill they are exposed in a large sand and gravel pit. At the base there are 2m of pebble and boulder gravel in a sandy matrix, topped by up to 20m of cross-bedded sand, incorporating pebble and cobble gravels. A belt of moraine forms a hummocky ridge that cuts across the Glenmornan valley from Ballymagorry in the west to Artigarvan in the east. There is a series of cross valley ridges, the highest reaching the 100m contour. They are dissected at Artigarvan by an east to west draining meltwater channel. The deposits lie immediately west of the flat-topped, remnant facets of valley fill already mentioned. The Strule and Mourne valleys show extensive evidence of a former valley-bottom fill in the form of a series of irregular, remnant facets on both sides of the valley from Strabane in the north to the northern edge of Newtonstewart in the south. In the upper reaches of the valley it approaches the 60m contour, descending progressively northwards to 10m at Strabane. Nowhere are these deposits exposed. Along the Derg River valley, particularly the north bank, an extensive belt of hummocky glacial deposits with frequent kettle holes, 11km long by about 1km wide, drapes the low ground. The upper level reaches between 60m and 70m and the sketchy exposure reveals glacio-lacustrine gravels and sands. A discontinuous, segmented arc of steep-sided, moraine deposits extends from Wood Hills to Newtonstewart, and at Laragh a face can be seen where the moraine has preserved the shape of its direct contact with the ice. To the south west there is more hummocky ground cut through by a dramatic meltwater channel, to a depth of 20m. The northern flank merges smoothly on to the 60m terrace already mentioned in the Strule valley. At the confluence of the Derg with the Mourne, at Millbrook, segments of a former moraine are preserved on both banks of the Mourne. The surviving ridges reach a maximum height of 20m and the flanking terrace deposits cover them in part. There is an associated deposit of sand to cobble grade, crudely stratified gravels. There is also a small hummocky area near Lisatunny House, at the head of a deeply incised meltwater channel draining northwards towards Douglas Bridge. Another meltwater channel, 2km long, forms the valley between Clady Hill and Whisker Hill. The lack of exposure in this entire area limits its interpretation and much is inferred or deduced from individual and associated landforms. The Foyle valley depression was a major route of ice flow when the glaciation was at its most severe during the Midlandian but towards the end of the stage, as the climate improved, the ice sheets began to waste and the northerly outflow ceased. The ice front then commenced a southerly retreat. At the same time, the ice in the Sperrins decayed and the ice front wasted westwards. A body of stagnant ice at New Buildings deposited its pulverised rock load on to the rock foundations, ridged and grooved by the earlier ice mass flowing north. As this was happening, the decaying margin of the ice sheet spread a canopy of outwash deposits in its wake near the intake to the Burngibbagh channel, obviously free draining at the time into the Faughan. The flat-topped facets of deposits in the Glenmornan valley are clear evidence of ice blocking its entrance leading to a build-up of meltwater. This lake had a maximum surface level of 160m above modern sea level, and the plentiful sediment washed into the lake along well-defined courses formed steep-fronted deltas in the relatively still water. The direction of flow appears to have been from the ice dam for much of this time. There are no substantial moraine deposits south and east of Artigarvan until the Deerpark ridges at Newtonstewart are reached, suggesting that the ice retreated rapidly over this stretch of ground. The ice sheet blanketing the area appears to have been flowing north to its melting fringe, and the deposits in the Derg valley suggest that ice flowed northwards across the pre-existing valley and melted south without ever flowing along the valley. This hummocky and kettle-holed deposit shows all the appearance of having collapsed on to stagnant ice. The ice contact feature associated with the Deerpark moraine shows the ice to have been on the southern side, and this and other moraines in the immediate area imply a period when flow and melt were in equilibrium for a period, allowing the deposition of the moraine in a confined belt. This was also the source of supply of sediment feeding the glacial lake occupying the lower Strule and Mourne valleys, where the remnant terraces imply a northward sloping series to Strabane. The moraines and terraces have been subjected to later erosion, extending into post-glacial times. Despite the lack of outcrop, the sequence of events during the melting of the ice sheet in the Foyle area and its progress southwards emerges with some clarity. The moraines and terraces in the vicinity of Deerpark, immediately west of Newtonstewart, are of exceptional quality and national importance.


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