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Fintona Hills Complex - Dungoran kame-esker-outwash systemFermanagh, Tyrone
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Summary Full report
Site Type: Inland exposure
Site Status:
District:
Grid Reference: H403565, H445595
Rocks
Rock Age: Quaternary
Rock Type: Gravel, Sand
Interest
Other interest: No data, Glacial deposit, esker, kame terrace, outwash

Summary of site:

Under the north-west slopes of the main Fintona Hill mass, south of Fintona, is an area of pronounced sand and gravel ridges with a north-east/south-west directional trend. The two main exposures, at either end of this elongate strip, reveal a late glacial phase of water coursing through ice tunnels in the retreating ice sheet and a spread of glacial sediments in shallow lakes in its wake.

The Crockroe outcrop at its western end exposes the internal structure of one of the ridges. The face is 12m long and not quite 3m high and reveals gravel beds 50-80cm thick, some with particles supporting each other, others suspended in a fine grained matrix. Interbedded with them are deformed, thinner beds of sand, clusters of cobbles and boulders. Imbricate structures in the coarser beds (like overlapping tiles, inclined downstream) show water flow to have been south-west to north-east. The Syonfin sand and gravel pit at the east end of the site has a face 50m long and 11m high. It reveals a sequence of sand and gravel beds 20-60cm thick with discontinuous, even-grained sand beds 10-30cm thick. The entire deposit is sealed beneath a coating of diamicton (a glacial paste containing particles of all sizes) up to 2m thick. The gravels are cross-bedded and cut by later channels and separated from the sands (only 10% of the exposed surface) by sharp boundaries. Some of the pebbles in the gravels are rocks from the Tyrone Igneous Complex, 15km to the north east. Cross bedded structures indicate flow to the east or slightly north of east. The Crockroe ridge exposure clearly indicates a sub-glacial esker origin, a tunnel within the ice sheet gushing with meltwater carrying all sizes of glacial debris from boulders to rock flour. It was flowing west to east and at some stage became largely blocked and finally choked by its load of rock debris. It finally subsided on to bedrock as the margin of the ice sheet retreated westward. The ice tunnels that remained open regurgitated their load at or near the margin of the ice sheet and water spread the sediment over the glaciated landscape for a considerable distance, in the form of drapes and fans, into and between constantly shifting marginal lakes. The deposits in the Syonfin pit could have originated in this way because the cross-bedding suggests a single channel or a closely related series of sources providing a steady supply of sediment from the west, probably when the edge of the ice sheet had retreated several kilometres from the immediate area. The diamict capping the deposit followed after the glacial sedimentation had ceased, probably derived from mobilised drift deposits on higher ground.


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