| Site Type: | Inland exposure, Pit | | Site Status: | ASSI | | Council area: | Derry City & Strabane District Council | | Grid Reference: | C4603 | | Bing maps: | 54.87305,-7.28344 | | Google maps: | 54.87305,-7.28344 | | Rocks |
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| Rock Age: | Quaternary | | Interest |
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| Other interest: | Deglacial sediments, Glacio-fluvial sediment, kettle-hole, meltwater channel, moraine, outwash |
| Summary of site: |
| Dunnamanagh is tucked into the Burn Dennet valley, about 1500m beyond the abrupt, southerly bend into its upper valley. The basin centred on this bend is the focus of a complex sequence of events, during and after the retreat of glaciers from the area in the final stages of Ireland’s last glaciation. | | Four main areas are described from their characteristic landforms: the area between Ardcame and Raspberry Hill; the mouth of the Altinghree valley, between Garvagh and Raspberry Hill; the terraces between Curradrolan Hill and Balix Hill, extending down most of the Burn Dennet valley; and the moraines around Lough Ash. | | A terrace banked against the southern slopes of Slievekirk curves east and south to the western flank of Eglish. It is now divided by the valley of Bonds Glen. Its highest point is 150m above sea level at its western end, progressively falling to about 100m in the south east. There are no exposures showing its internal composition. | | The area from Greaghan Hill to Raspberry Hill forms the Garvagh moraine, about a square kilometre of hummocky mounds, ridges that cut across the Altinaghree Burn and others that parallel the valley axis. At Garvagh the cross-valley ridges, about ten in number, achieve their maximum height of 20m. The burn drains northwards into the watershed separating the Burn Dennet and Faughan basins before turning abruptly west into Burn Dennet. East of the bend is a flat bottomed valley, tightening into Bonds Glen, flanked by small terraces on its southern side at the 90m contour, near Raspberry Hill. It drains into the Faughan, 5km to the north east. | | In the upper reaches of the Burn Dennet, south of Lisnaragh Bridge, a substantial moraine ridge up to 40m high and 20m wide forms the limit of the terrace complexes that extend down-valley in pairs on either side. Against the moraine the terrace reaches a height of 140m above modern sea level but then falls progressively to 75m at Dunnamanagh. Below the town there are two terraces: the upper at 95 m, falling to 52m about 5km downstream; and the lower from 65m to 20m at Milltown Burndennet, about 7km downstream. There are also terrace fragments close to modern river level and in the sand and gravel pits at Moyagh cross-bedded sands and gravels with cobble beds can be seen. | | In the bottom of the valley, about 1.5km north east of Dunnamanagh, between Fawney in the north and Bunowen in the south, is an area of mounds deeply pocked by kettle holes. An exposure in one of the mounds reveals pebble and cobble gravels in cross-cutting channels, topped by sands. Water flow directions in these deposits are west to north west. | | A ridge at the south-west end of Lough Ash is at the head of the Lough Ash meltwater channel, which is 10m deep and incised through the deposits into bedrock in places. It fed the Altinaghree Burn via Altdarragh Glen. | | The detail of the processes that created these deposits is sketchy at best, since they are so little exposed, and their interpretation and the proposed sequencing should be treated with some caution. | | The earliest phase appears to start with an ice lobe centred in the valley basin immediately north of Dunnamanagh. The terraces from Ardcame to Raspberry Hill appear to have formed on the margin of the ice mass against the bedrock of the area. They are considered to be either lateral moraines or kame terraces or both. Lateral moraines are accumulations of rock debris that formed when frost shattered valley walls and rocky material on the ice surface itself fed their debris into the depression between the ice and the valley wall. It is also in this zone that kame terraces formed by the creation of marginal troughs, as the reflected heat from the valley walls melted a linear depression against the ice, along which meltwater with its load of glacial debris flowed. These deposits became stranded as the glacier retreated. | | The deposits north of the Garvagh moraine appear to be outwash from the ice to the south. They were flushed into the centre of the basin, burying masses of static ice that later melted to form kettle holes. Many small bodies of standing water were a feature of the area at that time, rapidly draining and reforming as channels meandered across the surface, and there is some evidence for a period of reduced water. The lower Burn Dennet was still occupied by ice, blocking drainage to the west, so the outwash fed east and north east into Bonds Glen and the Faughan valley. | | As the climate continued to warm, ice retreated southwards from the Dunnamanagh basin and eastwards up the Burn Dennet valley. This ice front appears to have stabilised in the Lisnaragh area for a period, releasing the huge moraine spanning the upper valley there. In this period, the ice of the Foyle system that had been blocking the mouth of the Burn Dennet retreated south, so that drainage along the valley became possible. The terraces from the Lisnaragh moraine appear to be continuous with those in the lower Burn Dennet and date from the progressive wasting of the glacier. The moraine at Lough Ash and the kame terraces in the Moor Lough area were also deposited at this time but with drainage to the east, cutting the Lough Ash meltwater channel. | | The degree of meltwater flooding of this entire area was considerable, deeply eroding all the outwash and moraine materials. The modern landscape contains only remnants of what must have been previously much more extensive deposits. | | If this interpretation is accepted, the area is important for two main reasons. Its landscape documents the decay of the major ice sheet southwards towards the Sperrins. The moraines and their associated terraces show ice retreating east and south in the valleys forming lakes confined by ice pressure from the Foyle valley, and the sequence of events when that pressure was released and the mouth of the Burn Dennet became free draining. |
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