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Pomeroy ComplexTyrone
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Summary Full report
Site Type: Inland exposure
Site Status:
District:
Grid Reference: H26133726, H26823679, H27233663
Rocks
Rock Age: Quaternary
Rock Type: Gravel, Sand
Interest
Other interest: No data, Glacio-fluvial sediment, Glaciolacustrine deposit, Subaqueous outwash, Gilbert-type delta, meltwater channel, moraine, outwash, roche moutonnee, subaqueous fan

Summary of site:

The glacial deposits around Pomeroy tell a clear story of events as the regional ice sheets that blanketed the area at the height of the last glaciation (the Midlandian) began to decay and shrink back to the main glacial centres.

There are seven clusters of glacial features that provide the main evidence for events. The first is in and around the Camowen River between the highland masses of Cregganconroe to the north and the Fintona hills to the south. In this area a massive fan of glacial debris was deposited in the form of two cross valley ridges, the larger eastern one now deeply dissected, probably by meltwaters as the ice sheet decayed. At Ballynapoll the nature of the deposits can be seen in extensive exposures where they consist of vaguely bedded boulder, cobble and pebble gravels which progressively transmute in an easterly direction into beds of sand and cobble gravels. There is some small faulting in the deposits, created as they subsided under gravity into the meltwater channels. About 350 m north west of the first exposures, a second deposit can be seen in the flank of a terrace laid bare by collapse into an old meltwater channel. Here ripple-marked fine sands fill channels 10 to 15 m wide. The fabric of these deposits suggests that the water that carried the sediment was flowing to the south west. The second feature takes the form of a series of small ridges across the same valley, between Tremoge and Rockstown north east of Carrickmore, some armoured with boulders more than 3 m across. They consist of boulder and cobble gravels with some sand beds, one containing two large boulders. The third cluster is on the Altanagh River at the mouth of a depression, an area of steep-sided ridges, some up to 20 m high, running parallel with the valley sides. Three similar ridges can be seen further up the valley. The fourth feature takes the form of a substantial deposit (4 square kilometres) filling the bottom of the Camowen river valley, now dissected by the river. The fifth deposit is a flat-topped, steep-sided feature in the depression between Cranlome Hill and Lurganmore with two extensions to the south west in the form of ridges. It lies 9 km south west of Pomeroy on the southern edge of the Fintona hills in the headwaters of the Ballygawley Water. At the top of the valley the dead ground between the Ballygawley Water and the Owenbrack River is a flat plain obscured by peat. Near Crooked Bridge exposures show it to be channelled beds composed of sands and cobbles. Flow was broadly to the north west. East and south east of Pomeroy is the sixth deposit between Lime Hill (2 km north of the town) and Pomeroy Forest (immediately to the east). The rolling, hummocky terrain is best seen around Killey Bridge. The final group of features takes the form of dissected flat-topped terraces at Cranoge and Galbally, again on the south eastern fringes of the Fintona hills. These sites provide the evidence for a thinning and eventual parting of the ice sheet covering the entire area starting just north of the Pomeroy valley. The ice margin retreated southward into the Pomeroy valley before dividing into two lobes, one retreating east towards the ice mass centred on the Lough Neagh Basin, the other west into the other local centre in the Omagh Basin. Between the lobes, in the early stages of retreat, a substantial glacial lake formed (surface at 170 m above sea level) and began to fill with sediment swilled along the margins or within the ice by meltwater (Altanagh Bridge) to create an extensive sediment fan (the Ballynapoll Fan). As the ice damming this lake melted, the lake drained to the west cutting channels in its own deposits as it went. The sediments east of Pomeroy are reworked from this same lake. The eastern ice lobe retreated to the east and the moraine at Limehill marks a temporary period when the ice front stabilised, melting matching the rate of flow resulting in the release of mounds of glacial debris. The lateral moraines flanking the valley at Pomeroy Forest define the edges of the tongue of ice in the valley bottom. The ice on the Fintona hills retreated up the local valleys depositing moraine complexes (as at Altanagh Bridge) and in the final stages the glacial debris was swept out to create outwash plains in the valley bottoms (Owenbrack and Sluggan). With the much degraded ice margins now situated on the eastern edge of the Fintona hills and in the Omagh Basin, a large lake with a surface level of 245 m above sea level occupied the ice-free valleys and the massive amounts of glacially pulverized rock washed into the margins building extensive deltas (as between Cranlome Hill and Lurganmore extending to Edenfore). As this lake drained with further melt back of the margin towards Lough Neagh, a large number of smaller lakes survived for a time filling rapidly with local and reworked sediments from higher elevations (the deltas at Altmore and the deposits at Galbally). The landscapes of the Fintona hills have high aesthetic value and most of the deposits described are relatively untouched by gravel and sand working. A case could be made to offer the area some kind of landscape status.


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