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Glenshesk ValleyAntrim
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Summary Full report
Site Type: Inland exposure, Pit
Site Status: AONB
District:
Grid Reference: D31434343
Rocks
Rock Age: Quaternary
Rock Type: Gravel, Sand
Interest
Other interest: No data, Glacio-fluvial sediment, ice lobe, meltwater channel, moraine ridge, Nye-type channel, outwash

Summary of site:

In the valleys of the Glenshesk River and the Inver Burn, a suite of glacial deposits relates the closing events of the ice retreat from the area at the close of the final phase (the Midlandian) of the Pleistocene glaciation in Ireland,.

At the junction of the Inver Burn with the Glenshesk, there are two components of a ridge, revealing deposits (sands with convolutions, ripple marked surfaces, cross-bedding and beds of pebbles and cobbles) that were washed out from the margin of an ice sheet by torrents of meltwater. Slightly further up Glenshesk, at Doonfin, there is another ridge running for about 1km, between the 150m and 160m contours, roughly aligned with the valley of the Inver Burn. To its south west are three smaller ridges with a south-west/north-east orientation and their eastern parts are covered by the western edge of the Greenan Fan, described later. At Tenaghs, 500m further up Glenshesk, a strongly defined ridge (composed of sediments of various grain sizes with gravels below, topped by well-defined sand and gravel beds) is deeply gashed by a meltwater channel, now exploited by the Glenshesk River. This ridge is 15-20m high and its sediments are locally derived, largely basalts and schists. Immediately east of the valley there is an arcuate ridge on the flank of Oghtbristacree Mountain. The mid-section of the Glenshesk valley is dominated by a large fan-shaped deposit, the Greenan Fan, with its apex on the 180m contour in a 10m deep channel cut into the bedrock of Oghtbristacree Mountain. From here it descends and broadens down the valley to its flat-topped toe at Glenshesk Bridge. It is deeply incised by the Glenshesk River and its merged tributaries, the Greenan Water and the Owencam River. It is this fan that partly covers the ridges at Doonfin. Downstream and 3km north of the toe of the fan, remnants of earlier terraces can be seen, clinging to the sides of Glenshesk 10-15m above modern river level. The relationships of these landforms allow the deduction of a sequence of events during the glacial retreat. The main ice mass was flowing from the high ground to the south east into the valleys of the Inver Burn and the Glenshesk. Debris was washed across the surface of this ice and beyond into areas with detached and static ice masses that were eventually completely buried. The main ridges at Glenbank and Doonfin could be the last remnants of a terminal moraine banked against the snout of this smothered ice tongue that eventually retreated east and south east. Meltwaters were vigorous in this area and probably removed most of the loose glacial debris. As the ice melted and retreated, the stream sediments standing on the ice gradually sank on to bedrock to form the ridge and mound complex (kames). The melting of deeply buried ice led to the collapse of the enclosing sediment to form the kettle holes. The ridge on the west wall of the higher reaches of the Glenshesk valley beyond Doonfin is a remnant of stream flow and minor water ponding on the flank of the ice filling the valley. As the ice retreated further, there was a period of stability that caused debris, released by melting at the snout, to bank against it, forming the Tenaghs Ridge. Further melting and retreat released vast volumes of water that seriously modified the ridge during the later deglaciation. It was in this final deflation that the Greenan Fan was created. Torrents of meltwater were tightly channelled on to bedrock and excavated a grooved valley beneath the ice on the west flank of Oghtbristacree Mountain. The water carried a considerable load of all sizes of glacial debris that spread and veiled the bottom of the Glenshesk valley, immediately above its confluence with the eastern end of the Inver Burn valley. It partly conceals the earlier ridges and their associated features, firmly establishing that fan development was the final event in this area. The terraces in the lower valley at Drumeeny were probably formed as meltwaters progressively swept away deposits in the upper valley; later erosion has reduced them to the facets we see now. This area is entirely within a designated Area of Outstanding Natural Beauty and therefore receives some protection from development. Its late glacial and postglacial history as related by the landforms of the area is clear and important and a few of the features, such as the Greenan Fan, are of national importance. Its association with a channel incised into bedrock (a Nye or N-type sub glacial channel) is similarly significant. There has only been one site of intrusive sand and gravel extraction in the area, now inactive. The management of the area should ensure that these landscapes remain essentially undisturbed.


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