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Glenelly/Owenkillew Valleys (Gortin)Tyrone
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Summary Full report
Site Type: Inland exposure
Site Status:
District:
Grid Reference: X24933858
Rocks
Rock Age: Quaternary
Rock Type: Gravel, Sand
Interest
Other interest: No data, Glacio-fluvial sediment, esker, flat-topped delta, kettle-hole, moraine, Nye-type channel, subaqueous fan, ice contact

Summary of site:

The Glenelly and Owenkillew valleys and the Glenlark, a tributary of the Owenkillew, contain a wide range of related glacial and post-glacial deposits. They were laid down in the final stages of the last glaciation in Ireland (the Midlandian) at the close of the Pleistocene period, roughly 12,000 years ago, when the glacial ice was in final retreat.

The two main valleys are broadly parallel, trend east-west and are carved into the ancient Dalradian metamorphic rocks (mostly schists). They are confined by long, undulating ridges of high ground, important features in the glacial retreat, which are incised by meltwater overflow channels. Two breach the ridge on the northern side of the Glenelly River in the area north of Plumbridge. These are Butterlope Glen, with a base level of 210m above sea level, and the Letterbrat channel, with a base level at 180m. Both drain into the Inver channel to the north which, in turn, merges with the Burn Dennet drainage. The ridge between the Owenkillew and Glenelly valleys is breached by the Barnes Gap, between the summits of Mullaghbane and Mullaghbolig, with a base level of 250m above sea level, and at Sawelbeg at the head of Glenlark where the base level is 319m. Both appear to have drained north into the Glenelly valley. The Glenelly valley, over much of its length from just west of Mullaghgreenan to Lough Ouske on the high ground at the valley head, contains extensive deposits formed both under the ice and in glacial lakes. There are also lateral moraines clinging to the valley walls over this 20km stretch. The main deposits are the belt of steep moraine ridges up to 8m high, losing height to the south and west from the 330m contour to the 260m level. At Sawelbeg and Barnes Gap there are clear side-valley moraines and the main sediment deposits are wedges and fans, best seen at Barnes Gap, Goles Forest and Cranagh. Weak moraine ridges cross the valley in the vicinity of Goles Forest but at Dergbrough, 2km west of Plumbridge, where the valley opens out, there is a strong cross valley moraine 15m high which has diverted the river to the north for 300m to a breach before it resumes its westward flow. The Owenkillew and its tributary, Glenlark, are also liberally decorated with glacial retreat features and deposits. From its confluence with the Finarrow Burn to the head of Glenlark, a series of cross valley moraines reaches up to the 290m contour on the valley walls. Lough Lark is confined by hummocky mounds to the south west and it drains along the old meltwater channel northwards, spilling down the southern slope of the Glenelly valley to join its flow. There are further cross valley moraines at Fallagh Lower on the Owenreagh River, 3km upstream of its confluence with the Owenkillew, and a substantial and pronouncedly arcuate moraine, 4km long, extends from Cosh House through Moyle Glebe to Milltown, across the Owenkillew, 1km upstream of Newtownstewart. The most obvious and striking feature on the Owenkillew is a landform about 1.5km south of Gortin village. It consists of layered sediments at the mouth of Altavaran Glen to the south. Groups of hummocks and discontinuous ridges on a flat plain slope northwards where they meet the steep southern slopes of what appear to be eroded fragments of a higher set of deposits. There are three distinct levels in this complex, the highest at 256m above modern sea level and dotted with seven small lakes. The others are at the 210m and 180m contours. They are deeply dissected by north-draining channels and the north faces of the deposits are steep and fairly flat. The sediments can be seen in the flanks of eroding segments and are typical of glacial lake deltas - mostly cross-bedded sands, gravels and cobbles. Kettle holes (flooded depressions created by the melting of large masses of buried ice), evidence of a torrent of meltwater below the ice feeding debris to the delta, choked ice tunnel deposits (eskers), sediments that were in contact with the ice and sediments deformed by moving ice can all be seen here. Despite the poor exposure of the internal composition and structure of most of these landforms (leading to some uncertainty of their nature), a rough sequence of events can be proposed for the area. The original ice cover of the entire area was derived from the Omagh basin to the south west. At the height of its development, this major ice sheet flowed north east across the region and was so thick that it was uninfluenced by the east-west trending valleys of the Owenkillew and Glenelly. The Barnes Gap and Altavaran Glen, both deeply incised into bedrock, were probably formed at this time by powerful meltwater torrents coursing beneath the ice. As the climate improved and the ice sheet began to deflate, the underlying rock topography exerted an influence and split the ice along the Slievemore to Slieveavady ridge, separating ice masses in the Glenelly valley to the north and the Owenkillew in the south. The main ice sheets were still centred on lowland depressions but the ridges and valleys now controlled events. The entrance of the Glenelly valley was blocked by ice to a level above the bases of Butterlope Glen and the Letterbrat channel. As the ice tongues that occupied the valleys began to melt and retreat down gradient to the west, they left lateral moraines banked against the valley sides. Ice also blocked the mouth of the Owenkillew (evidenced by the moraines at Crosh, Fallagh Lower and the upper Owenkillew) and prevented the escape of meltwater to the west, leading to a build-up stretching all the way up the valley. It was into this extensive lake that the complex of sediments south of Gortin village was deposited. The initial lake level (the surface of the topmost delta) was at 256m above modern sea level but when the Butterlope Glen channel (base at 210m) became ice-free, the lake rapidly drained to the 210m surface, preserved at Gortin; as further ice wastage opened the Letterbrat channel (base 180m), the process was repeated, giving the last lake level for which evidence survives, at 180m. The final melting left the foundations of the modern landscape and the chief changes since have been largely restricted to the erosion of the unconsolidated glacial and lake sediments. This area has largely escaped the exploitation of sand and gravel deposits which has pocked and defaced the glacial record elsewhere, so its scientific value remains largely intact. It is also an area of great character and high aesthetic quality and is potentially a future national park. It follows that the sensitive management of these landscapes is of national importance and all proposed developments will require careful scrutiny for their environmental impact.


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