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Cumber Delta, Dungiven BasinLondonderry
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Summary Full report
Site Type: Inland exposure
Site Status:
District:
Grid Reference: X25444074
Rocks
Rock Age: Quaternary
Rock Type: Gravel, Sand
Interest
Other interest: No data, Deltaic sediments, Glacio-fluvial sediment, Glaciolacustrine deposit, braided gravels, deltaic foresets, deltas, delta

Summary of site:

Deglaciating landscapes (where regional glaciers were rapidly melting and in full retreat) change rapidly, leaving a litter of confusing deposits in their wake. It is difficult to imagine Northern Ireland in such a state but at the end of the last glaciation, the Midlandian, the area resembled parts of modern day Alaska and Greenland where the current episode of global warming is again deflating ice sheets and creating almost identical conditions. Such landscapes are smothered in deposits – boulder clays, moraines, eskers, drumlins and erratic blocks, all lying on planed and grooved bedrock.

This site around Cumber, less than a kilometre south of Claudy, shows another aspect of glacial retreat where pockets of ice lingered in the valleys of the area while the ice sheet retreated southwards into the high and bleak country of the Sperrins. Torrents of meltwater surged northwards from the main ice fronts but the valleys they drained into were blocked by stranded masses of ice, effectively dams. The story of Cumber is of a lake that formed behind first one, and then a second such dam and the rapid creation of a massive delta as the floods of meltwater constantly washed the enormous banks of exposed and released pulverized rock into it. About 1km south and slightly east of Claudy is the steep northern slope of a flat topped feature extending south and east for 4km towards Straid Hill. Sand and gravel quarries have gnawed into its western and eastern flanks, where the deeply incised Faughan River (in the east) and the Glenrandal River (in the west) have also laid the deposits bare. The Glenrandal has cut its valley into the top of the delta, isolating a mass on the western fringe now being worked in Robinson’s Quarry. The deposits exposed tell a fairly consistent story. Where excavations have penetrated the full depth of the delta, two units can be seen. The lower consists of 1.4m of coarse, bedded sands with some fine laminations (beds less than 1cm thick), a few with rippled surfaces. The upper unit has eroded the lower, forming a surface of discontinuity topped by coarse, cross-bedded pebble and cobble conglomerates and gravels. The cross-bedding is interrupted by later channels, some cutting through the discontinuity into the lower unit. These channels are filled with water sorted beds of pebbles and cobbles showing the force of currents passing through. In the west of the area the channels can be 20m across and 8m deep. No laminated sands are seen in the quarry on Gilky Hill where cobble, pebble and gravel deposits dominate, cut by channels up to 25m wide and 10m deep which are choked with the same kind of deposits. The clasts divide into two roughly equal rock types: schists and white vein quartz, both locally derived. The sequence of events deduced from the evidence in the area starts with the continental ice sheet retreating south into the high Sperrins as the climate warmed. Stranded ice blocked the Faughan valley in the area south east of Londonderry city while the northerly retreating Scottish ice sheet blocked an alternative meltwater escape route near Limavady. The result was the creation of a glacial lake in the Faughan valley. As the meltwater poured into the expanding body of water the coarse material settled inshore while finer material was carried further out by a progressively enfeebled current to settle in fine layers (unit 1). The inshore lake margin rapidly silted up, with coarse material quickly reaching the surface water level; this forced the streams to cross a rapidly expanding delta as more coarse material settled almost immediately on reaching the lake. Violent flow in these braided streams cut deep into the earlier deposits, opening channels which themselves became choked as the water changed course. A period of cold leading to ice stagnation followed, with ice forming hummocks behind the lake while in the lake itself large masses of ice, almost mini icebergs, grounded on the lake bed and were then progressively buried by later sands and gravels. (These large, enclosed ice masses survived for protracted periods while the sediments around them consolidated so that when the ice eventually melted large water-filled holes, called kettle holes, dotted the landscape.) The ice then recommenced its retreat south and the lake shore migrated with it forming the deposits at Gilky Hill. That was the last act in the formation of the delta; sometime later the ice dam failed, most likely in the Faughan valley, thereby stranding the delta in its present land-locked situation. The deltaic landform, with its exposures of typical deltaic deposits, combined with evidence of variable lake levels, the damming effects of Scottish ice and a late intensification of cold conditions, all provide considerable interest and suggest much future research potential. Sand and gravel deposits in areas of high aggregate demand such as this are always under threat and this still fairly untouched deposit will degrade if quarrying continues or intensifies. Equally, if it ceases, exposed sections will weather and become obscured.


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