ESCR - Dunnamanagh Complex

Earth Science Conservation Review

Summary Full report
Dunnamanagh Complex	Tyrone

Site number:	473
ESCC:	IS
Locality Type:	Inland exposure, Pit	Status:	ASSI
Grid Reference:	C4603
Google maps:	54.87305,-7.28344
County:	Tyrone	Council area:	Derry City & Strabane District Council
Period:	Quaternary

Site Description
Highlights:
The Dunnamanagh complex, located in a structural basin and in the western valleys of the Sperrin mountains, demonstrates the features of a deglaciated landscape comprising extensive glaciofluvial and ice-marginal valley fill followed by meltwater incision. Most landforms are relatively intact, free from commercial sand and gravel extractions and provide examples of multi-tiered terracing, meltwater incision and hummocky topography which are important at a regional level. The complex records ice backwastage south and southeastwards from the Faughan valley, Co. Londonderry towards the Sperrin highlands and westwards into the Foyle valley lowlands. Constraints imposed by bedrock topography during recession strongly influenced ice marginal configurations with morainic belts, meltwater and therefore glaciofluvial deposits focused along the axes of the main river valleys.
Description:
There are four principal landform assemblages in the Dunnamanagh complex: 1. Ardcame Morainic Complex A large (2km2) arcuate belt of gently sloping to flat-topped topography occurs against the lower slopes of Slievekirk and Eglish mountains, extending from Raspberry Hill (247300 404900) to Ardcame (244400 405100) across the Bond's Glen meltwater channel. The bench-like feature decreases in height from 150m O.D at Taboe Glebe (244800 406200) to approximately 100m O.D. at Ballycallaghan House (248100 405400) where it forms a 0.75km long hummocky ridge occurring along the southern intake of the Bond's Glen channel. No exposures are present in the deposit. 2. Garvagh Moraine and Raspberry Hill terraces The area between Garvagh (246400 403700) and Liscloon (247300 403700) consists of a 1km2 belt of small-scale hummocks, cross-valley and valley parallel ridge segments in the topographic low between Creaghan Hill (245500 403200) and Eglish (250400 404300). Cross-valley (NE/SW) ridges reach a maximum height of 20m at Garvagh, on the western edge of the belt, with approximately ten others of up to 7m high within the valley bottom immediately to the east and northeast. No exposures occur, and stratified drift is absent from the remainder of the Altinaghree valley to the southeast. The Altinaghree Burn is diverted northwards by the Garvagh complex along a small-scale meltwater channel. This joins the deep, steep-sided, flat-bottomed Bond's Glen channel, which decreases in elevation northeastwards towards the lower Faughan river valley. Immediately north of Raspberry Hill several small-scale, flat-topped terrace fragments occur at about 90m O.D. subparallel to the axis of the Bond's Glen channel. 3. Lisnaragh Moraine and Burndennet Terraces A large-scale cross-valley ridge up to 30-40m in height and 20m in width occurs in the upper Burndennet river valley at Lisnaragh (246800 400200). The ridge is 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 (244800 400500), to 80 and 75m O.D. immediately southwest of Dunnamanagh town. Two main flat-topped terraces occur along the lower Burndennet valley from northwest of Dunnamanagh (244200 403200) to Milltown Burndennet (239300 404400). The upper terrace decreases from 95m O.D. at Dunnamanagh to 52m O.D. at Stranabrosny (242000 404800). The lower terrace decreases from 65m O.D. at Corbylin Cottage (244000 404000) to 20m O.D. at Milltown Burndennet. Smaller terrace fragments occur approximately at the same elevation as the Burndennet river, which now occupies the steep-sided meltwater channel, which dissects both terrace levels. In the lower terrace a partially grassed exposure (5m deep at most) in a commercial sand and gravel pit at Moyagh (240400 404700) contains three LFAs. At the base LFA1 consists of interbedded large scale planar cross-bedded (dip 22°) sand and pebble to cobble gravel. Palaeoflow, based on cross-bedding orientation, was to the northwest (330-35°N). The LFA contains a wedge shaped unit of 5m of horizontally and ripple bedded well-sorted sand which tapers eastward over 30-40m into overlying cobble gravels. Both LFA 1 is separated by sharp contacts from the overlying LFA 2 which consists of up to 1m of crudely imbricated, poorly-sorted cobble gravel. Palaeoflow, based on imbrication was to 360°N. The 100m O.D. flat-topped terrace on the eastern flank of the Burndennet ends at Dunnamanagh town. To the northeast of the town limits, on the eastern banks of the river and northern slopes of Creaghan Hill, occupying the floor of the Dunnamanagh basin around Fawney and Bunowen, the topography is hummocky and deeply kettled. An exposure in the hummocky topography around Fawney/Bunowen (244900 404200) shows two principal lithofacies associations (LFAs). LFA1 consists of large-scale, trough cross-bedded pebble to cobble gravel occurring in cross-cutting channels up to 2m in depth. Palaeoflow based on channel orientation and cross-bedding was to 340°N. LFA2 occurs as a wedge-shaped body in sharp contact with LFA1 and consists of planar cross-bedded sand interbedded with laterally (northwards) thinning pebbly sand. Palaeoflow was to 270°N. Clasts consist predominately of schist and limestone (>80%) with small proportions (<1%) of pink granite. 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 Loughash meltwater channel reaches 10m in depth and is partially cut into solid rock. It conducted meltwater northeastwards towards the Altdarragh Glen (249800 402700) but is not associated with any obvious fluvioglacial deposits.
Importance:
The deposits post-date ice decay in areas to the east in the Faughan valley and Dungiven basin. They demonstrate the direction of ice decay was to the south and southwestward towards the Sperrin uplands when ice-marginal configuration controlled largely by bedrock topography. The Dunnamanagh complex records the two main morainic belts related to recession of ice-margins through the basin clearly indicating ice pressure from the direction of the Foyle valley. It also contains a provincially important example of a deglacial landform assemblage, the Lisnaragh moraine and lower Burndennet glaciofluvial outwash terraces.
Interpretation:
The lack of exposures prevents a detailed palaeoenvironmental reconstruction of the full range of deglacial palaeoenvironments. The general interpretation of the deglacial pattern, especially reconstructed ice-marginal positions in the area is inferred mainly from the relative location of landform assemblages, glaciofluvial terrace distributions and comparison with known patterns of ice wastage in adjacent areas to the north, east and west. 1. Ardcame Moraine Complex The well-defined apron of sediment banked along hillsides on the northern and eastern margins of the Dunnamanagh depression from Ardcame to Raspberry Hill is inferred to represent ice-marginal morainic and kame terrace deposits of an ice lobe centred on the Dunnamanagh basin. No exposure allows an examination of the sediment composition. The deposits pre-date the other sub-units in the complex as the ice margin is inferred to have retreated generally southwards from the Faughan valley. The Bond's Glen channel later conducted meltwater from ice in the Dunnamanagh depression northwards into the Faughan valley that may have been cut earlier as a subglacial conduit. 2. Garvagh moraine and Raspberry Hill terraces The cross-valley 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. The low-angle planar cross-bedded sand and pebbly sand (LFA2) beds (245300 404500) are interpreted to be low-angle foresets deposited into a localised waterbody located within the kettled outwash. The foresets are bordered by sharp contacts showing they were deposited after a hiatus in ice-marginal deposition probably associated with erosion by the reforming regional drainage network. The foresets exposed occur therefore probably as a localised unit of well-sorted sediment. The lower Burndennet was not ice free at this stage, with proglacial drainage of the basin directed along the Bond's Glen channel. 3. Lisnaragh Moraine and Burndennet Terraces Further retreat of the ice margins southwards from the Dunnamanagh basin was controlled by the bedrock topography of the region, with ice retreating up the Burndennet valley in the south of the basin. The formation of the Black Park (244600 402500) and Tirconnelly Glen (246700 403000) meltwater channels, which slope eastwards could have occurred at this stage or be related to earlier subglacial meltwater flow. Either a reduction in ice 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 (246700 400100). Terrace levels show that drainage of meltwater from the ice margin was focused northwards and then westwards along the course of the lower Burndennet river. The flat-topped terraces slope down valley from Lisnaragh and seem to be continuous with the terraces in the lower Burndennet valley. At this stage thick ice in association with the Foyle glacier must have withdrawn south of the Burndennet exit to allow proglacial meltwater to escape north along the Foyle system. At this time the morainic ridge impounding Lough Ash and the kame terrace, in which Moor Lough is situated, were also deposited, associated with drainage of meltwater to the northeast along the probably subglacially initiated Loughash channel. The deep dissection of the outwash fill in the lower Burndennet, the dissection of the morainic belt at Lisnaragh and the multiple examples of meltwater channels in the area indicates that the region was subject to meltwater floods during later stages of deglaciation, as ice withdrew further south. Some (probably flood) drainage also occurred along the Bond's Glen channel dissecting an outwash plain formed at 90m O.D. along the valley axis.
Notes:
For general information and references on the sand and gravel landscapes of N. Ireland, and for directions to other regional sand and gravel sites, see the following site: Key Site 439 - Introduction to the Sand and Gravel Landscapes of N. Ireland

Keywords
Relations:	Deglacial sediments, Glacio-fluvial sediment
Geomorph:	kettle-hole, meltwater channel, moraine, outwash

Access
Management:
The unit contains many excellent examples of closely associated deglacial depositional and erosional landforms in relatively good condition. Along the length of the Burndennet river mature woodland occurs on the steep meltwater channel slopes incised into the intact valley side flat-topped terraces. The hummocky terrain at Garvagh forms an example of distinctly ice-contact topography. The impeded drainage and the kettled area immediately northeast of Dunnamanagh provide a distinctly chaotic pattern of hummocks with contrasting slopes and topographic contrasts. The raised bogs developed in the kettle hollows are now under threat from disruption of the landscape through the expansion of commercial sand and gravel extractions. Extensive commercial quarrying of the deposits in the Dunnamanagh basin has not occurred and the possibility exists for landscaping the pit at Moyagh and washing plant northeast of Dunnamanagh. The stratified deposits are up to 50m in depth around Dunnamanagh and this huge reserve is potentially suitable for large scale extraction. Clearly such operations could seriously disrupt the skyline contours and current rural continuity of the valley fill vistas.

Access

Management:

The unit contains many excellent examples of closely associated deglacial depositional and erosional landforms in relatively good condition. Along the length of the Burndennet river mature woodland occurs on the steep meltwater channel slopes incised into the intact valley side flat-topped terraces. The hummocky terrain at Garvagh forms an example of distinctly ice-contact topography. The impeded drainage and the kettled area immediately northeast of Dunnamanagh provide a distinctly chaotic pattern of hummocks with contrasting slopes and topographic contrasts. The raised bogs developed in the kettle hollows are now under threat from disruption of the landscape through the expansion of commercial sand and gravel extractions. Extensive commercial quarrying of the deposits in the Dunnamanagh basin has not occurred and the possibility exists for landscaping the pit at Moyagh and washing plant northeast of Dunnamanagh. The stratified deposits are up to 50m in depth around Dunnamanagh and this huge reserve is potentially suitable for large scale extraction. Clearly such operations could seriously disrupt the skyline contours and current rural continuity of the valley fill vistas.

References (2)
Charlesworth, J.K., 1924. The glacial geology of the north-west of Ireland Proceedings of the Royal Irish Academy, vol. 36 pt. B, pp.174-314.
Colhoun, E.A., 1972. The deglaciation of the Sperrin Mountains and adjacent areas in counties Tyrone, Londonderry and Donegal, Northern Ireland Proceedings of the Royal Irish Academy, vol. 72 pt. B, pp.91-147.

Record type:	ESCR report
Entered by:	E M Porter
Last updated:	6 Jun 2005

Enlander, I., Dempster, M. & Doughty, P., 2026. Dunnamanagh Complex, County Tyrone, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=473. Accessed on 2026-11-06.