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Clogher ValleyTyrone
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Summary Full report
Site Type: Inland exposure
Site Status:
District:
Grid Reference: H630575, H386406, H372330
Rocks
Rock Age: Quaternary
Rock Type: Gravel, Sand
Interest
Other interest: No data, Glacial deposit, Glacio-fluvial sediment, Ice retreat, Ice stagnation, esker, moraine ridge

Summary of site:

The Clogher Valley was vitally important in the dynamics of ice flow and retreat in the last glacial phase, the Midlandian, of south-west Northern Ireland. Early in the stage part of an ice sheet centred on Omagh spread over the Fintona Uplands to the south, into the Clogher Valley and continued south along the line of the Upper Lough Erne basin. By the middle of the period this ice centre had grown considerably and it was migrating aggressively to the south, overwhelming the Fintona Uplands, the Clogher Valley and even the Slieve Beagh Uplands. Towards the end of the stage, the ice sheet to the east (centred on the Lough Neagh Basin) became dominant with strong radial flow but particularly to the west and south west, eclipsing the waning Omagh ice centre. This account describes the final retreat of the ice westwards along the line of the Clogher Valley, as reflected in the landforms of the area.

The valley is drained to the south west by the Colebrook and Tempo Rivers and to the east by the Fury and Blackwater. The solid geology underlying the superficial deposits is Carboniferous, mainly the Clogher Valley and the Ballyshannon Limestone Formations dating from the early part of the period, with the later Meenymore Formation on the southern flank across the Clogher Valley Fault. Devonian Old Red Sandstone forms the northern flank in the Ballygawley, Augher and Clogher areas to the north east. It should be said at the outset that, for an area of this size, from Lisnaskea and Brookborough in the south west to Ballygawley in the east, taking in the whole valley between, there is so little exposure of superficial deposits and such outcrops as can be seen are so widely separated, that any interpretation of events is almost totally reliant on landforms. For the purposes of description, the valley is divided into four parts, each differing from the others. Division one is in the narrow north-east end of the valley, in the area between Ballygawley and Augher. It is characterised by elongate ridges, 600m long by 200m wide and up to 30m high, aligned along the valley. They are breached in places and occasionally form enclosures or ‘rings’ in which later raised bogs evolved. On the northern flank, adjacent to the Fintona Hills, there are at least two spreads of sands and gravels. The ridges are believed to be moraines, marginal to the ice sheet as it dwindled into a valley glacier here and retreated to the south west. Typically with decaying ice fields, large bodies of ice become detached and stagnant and with active sediment flow around and partly burying them, they could eventually leave the ‘ring’ structures as the ice finally melts. Local ice or moraine dams on the north side of the valley probably account for the sands and gravels that filled the temporary lakes. Division two is the area continuing from the first between Augher and Mullaghmore, 2km south west of Clogher. There are two subdivisions, sectors A and B in the full account. The first is characterised by low undulatory hills with flat tops. They are small, up to 400m long, 200m wide and 20m high, and consist of glacial lake sediments. On land below the 100m contour they are steep-sided and surrounded by modern flood plain alluvium. On higher ground they are gentler and the deposits can be seen to rest on both bedrock and glacial drift. Only one site of any significance exposes their interiors - an old sand pit at Drumhirk where typical glacial sediments, such as stacked sands, silt and clay drapes and gravel occur. Sand wedges are present, their tops covered by later redeposited glacial debris. There is some structural displacement along normal and reverse faults. Sector B is dominated by massive ridges up to 1.2km long, 200m wide and 30m high, consisting of glacial debris and aligned along the valley axis. The best preserved are steep-sided and associated with channels presumed to have been cut by meltwater. The deposits of the first sector were formed in small lakes dammed by ice or glacial debris. The sediments appear to have been carried in at intervals by pulses of meltwater and settled to the lake bed. The process appears to have been steady and there is little indication of rapid, violent flow of large volumes of water. Flow directions determined from the sediment suggest that at least some of the sediment was derived from the Fintona Hills on the north west side of the valley. The sand wedges are casts of ice wedges formed when the ground froze solid to depth, cracked and filled with surface water that then froze. When the ice melted, the wedge shaped cavity was filled with layers of sand. The third division continues west from the second as far as Brookborough, a distance of 18km. The strong distinguishing features of this entire area are sand and gravel ridges up to 20m high and 2km long, again aligned along the central axis of the valley. They are steep sided, branching both up and down valley and have fairly flat tops. In places they appear to be standing on glacial drift material. Glacial lake sediments flank the main axis of the valley and also lie on top of glacial drift. The ridges are interpreted as eskers - the products of tunnels in the glacier through which meltwater and its associated load of gravel, cobbles and boulders coursed. The tunnels often choked on their load and the water found, and quickly expanded, new tunnels which, in turn, would also choke, hence the downstream bifurcations. Active tunnels often merged and filled, explaining upstream bifurcations. When the glacier finally melted these debris packed tunnels were dropped on to the landscape in the form of the ridges now named eskers. The lake sediments were probably part of the same process, as the water spread in front of the retreating ice. The fourth division continues down valley beyond the third, as far as Lisnaskea. In this area there are discontinuous mounds of sand and gravel standing on bedrock and what appears to be a thin skin of glacial drift, and there is a separate area of isolated drumlins. Deep channels cut by meltwater are a feature of the sand and gravels. There is a single fan of sediment in the area that appears to have been fed from an ice tunnel in the retreating glacier, a concept supported by local eskers. The deposits in the Clogher Valley show extreme complexity and a clear interpretation is further hindered by lack of exposure (imposing heavy reliance on surface morphology), possible extensive modification by meltwater channelling (degrading the original features) and a low gradient giving no clear indication of directions of water flow. These should be borne in mind in the following tentative attempt to sequence events. A north-east/south-west ice flow predominated in the last stages of the Midlandian, the final Irish glaciation, partially fed from the Omagh Basin. In and below the glacier a number of processes were operating simultaneously, including channelling by meltwater associated with eskers and the shaping effects of the ice on the ground moraine to form drumlins and rogan moraine features in the west. The hydraulic gradient, generally to the south west, changed temporarily at least once to form the Kilgreen sand and gravel deposits. The ice margin then commenced its retreat to the west with a period of standstill when the Clogher moraine was deposited. Further adjustments of the margin, followed by the formation of lakes behind temporary dams, led to the deposition of the glaciolacustrine sediments at Drumhirk. Following the draining of the lakes, the sediments were dissected by meltwater into hill forms. The retreat continued west and south and there is evidence to suggest that the southern limits of the ice sheet were migrating northwards leading eventually to stagnation and the gradual final melting to give the foundation of the modern landscape. Subsequent modification appears to have been largely the spread from the rivers of modern alluvium and the development of raised bogs in some parts of the valley. Without further exposure or an extensive borehole programme it is difficult to see how the many outstanding problems of this important area’s glacial and retreat features can be resolved.


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