Earth Science Conservation Review
| Donemana Complex - site of local interest | Londonderry |
| Site Type: | Mountains, Valley |
| Site Status: | local interest |
| Council area: | Strabane District Council |
| Grid Reference: | C4603 |
| Google maps: | 54.87305,-7.28344 |
| Rocks | |
|---|---|
| Rock Age: | Quaternary |
| Rock Type: | Gravel, Sand |
| Interest | |
| Other interest: | Glacio-fluvial sediment, cross-valley moraine, hummocky moraine, kame terrace, meltwater channel, moraine ridge, outwash terrace, deglacial |
Description:
There are four main principal landform assemblages in the Donemana complex:
1. Ardcame Morainic Complex - a 2km2 arcuate belt of gently sloping to flat-topped topography abutting the lower slopes of Slievekirk and Eglish Mountain, extending from Raspberry Hill [C473 049] to Ardcame [C444 051] across the Bond's Glen meltwater channel.
2. Garvagh Moraine and Raspberry Hill terraces - a 1km2 area between Garvagh [C464 037] and Liscloon [C473 037] consisting of small-scale hummocks, cross-valley and valley-parallel ridge segments. Cross-valley ridges reach their maximum height of 20m at Garvagh with approximately 10 others up to 7m high within the valley bottom immediately to the east and northeast.
3. Lisnaragh Moraine and Burndennet Terraces - a large-scale cross-valley ridge up to 30-40m high and 20m across occurs in the upper Burndennet river valley at Lisnaragh [C468 002]. The ridge has an abrupt contact down valley with the upper margin of an 8km long series of deeply dissected remnant valley-side terraces which can be traced along the course of the Burndennet river. The terraces are paired and decrease in elevation from 140m O.D. at the northern margin of the Lisnaragh ridge to 100m O.D. near Rousky [C448 005], to 80m and 75m O.D. immediately southwest of Donemana.
4. Lough Ash Moraine - a second, topographically prominent ridge impounds the southwestern end of Lough Ash. To the northeast of the lough, the steep-sided Lough Ash meltwater channel reaches 10m in depth and is partially cut into solid rock. The channel conducted meltwater northeastwards towards the Altdarragh Glen [C498 027] but is not associated with any obvious fluvioglacial deposits.
The lack of exposure prevents a detailed palaeoenvironmental reconstruction of the full range of deglacial environments. The general interpretation of the deglacial pattern is inferred mainly from the relative location of landform assemblages, glaciofluvial terrace distributions and comparison with known patterns of ice wastage in adjacent areas. The Ardcame Moraine Complex is inferred to represent ice-marginal morainic and kame terrace deposits of an ice-lobe centred on the Donemana Basin. The ridge segments and hummocky topography at Garvagh probably consist of morainic material deposited in a proglacial belt. Deposition of glaciofluvial outwash into the centre of the basin (around Fawney and Bunowen) occurred over and between blocks of dead ice located on the basin floor, with the formation of kettle holes and ponding of localised waterbodies. Further retreat of the ice margins southwards from the Donemana basin was controlled by the bedrock topography of the region, with ice retreating up the Burndennet valley. Either a reduction in ice-margin retreat rate or an increase in sediment supply to the ice-margin resulted in the formation of large cross-valley morainic accumulations in the upper Burndennet valley at Lisnaragh. At this time, the morainic ridge impounded Lough Ash and the kame terrace, in which Moor Lough is situated, was also deposited.
The deposits post-date ice decay in areas to the east. They demonstrate that the direction of ice decay was to the south and southwestwards towards the Sperrin uplands when the ice-marginal configuration was controlled largely by bedrock topography.
1. Ardcame Morainic Complex - a 2km2 arcuate belt of gently sloping to flat-topped topography abutting the lower slopes of Slievekirk and Eglish Mountain, extending from Raspberry Hill [C473 049] to Ardcame [C444 051] across the Bond's Glen meltwater channel.
2. Garvagh Moraine and Raspberry Hill terraces - a 1km2 area between Garvagh [C464 037] and Liscloon [C473 037] consisting of small-scale hummocks, cross-valley and valley-parallel ridge segments. Cross-valley ridges reach their maximum height of 20m at Garvagh with approximately 10 others up to 7m high within the valley bottom immediately to the east and northeast.
3. Lisnaragh Moraine and Burndennet Terraces - a large-scale cross-valley ridge up to 30-40m high and 20m across occurs in the upper Burndennet river valley at Lisnaragh [C468 002]. The ridge has an abrupt contact down valley with the upper margin of an 8km long series of deeply dissected remnant valley-side terraces which can be traced along the course of the Burndennet river. The terraces are paired and decrease in elevation from 140m O.D. at the northern margin of the Lisnaragh ridge to 100m O.D. near Rousky [C448 005], to 80m and 75m O.D. immediately southwest of Donemana.
4. Lough Ash Moraine - a second, topographically prominent ridge impounds the southwestern end of Lough Ash. To the northeast of the lough, the steep-sided Lough Ash meltwater channel reaches 10m in depth and is partially cut into solid rock. The channel conducted meltwater northeastwards towards the Altdarragh Glen [C498 027] but is not associated with any obvious fluvioglacial deposits.
The lack of exposure prevents a detailed palaeoenvironmental reconstruction of the full range of deglacial environments. The general interpretation of the deglacial pattern is inferred mainly from the relative location of landform assemblages, glaciofluvial terrace distributions and comparison with known patterns of ice wastage in adjacent areas. The Ardcame Moraine Complex is inferred to represent ice-marginal morainic and kame terrace deposits of an ice-lobe centred on the Donemana Basin. The ridge segments and hummocky topography at Garvagh probably consist of morainic material deposited in a proglacial belt. Deposition of glaciofluvial outwash into the centre of the basin (around Fawney and Bunowen) occurred over and between blocks of dead ice located on the basin floor, with the formation of kettle holes and ponding of localised waterbodies. Further retreat of the ice margins southwards from the Donemana basin was controlled by the bedrock topography of the region, with ice retreating up the Burndennet valley. Either a reduction in ice-margin retreat rate or an increase in sediment supply to the ice-margin resulted in the formation of large cross-valley morainic accumulations in the upper Burndennet valley at Lisnaragh. At this time, the morainic ridge impounded Lough Ash and the kame terrace, in which Moor Lough is situated, was also deposited.
The deposits post-date ice decay in areas to the east. They demonstrate that the direction of ice decay was to the south and southwestwards towards the Sperrin uplands when the ice-marginal configuration was controlled largely by bedrock topography.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Donemana Complex - site of local interest, County Londonderry, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=724. Accessed on 2024-12-26 |
| Previous Site | Next Site |