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Earth Science Conservation Review
| Lough Fea | Londonderry, Tyrone |
| Site number: | 477 | ||
| ESCC: | IS | ||
| Locality Type: | Inland exposure, Pit | Status: | TBC |
| Grid Reference: | H27993879,H27483855 End(s) of linear site, Not Entered | ||
| Google maps: | 54.29679,-7.57148 | ||
| County: | Londonderry, Tyrone | Council area: | Fermanagh & Omagh District Council |
| Period: | Quaternary | ||
| Site Description | |
|---|---|
| Highlights: | |
The deposits of the Lough Fea platform record the phased decay of an ice lobe from this upland structural depression and the strong influence of bedrock topography on the patterns of local ice marginal retreat. Ice marginal ridges and kame terraces mark the successive positions of a downwasting and backwasting ice mass. The ice, subject to bedrock control and ice pressure, split during the process of retreat to form two discrete ice lobes. Sediments of the western part of the glaciofluvial complex were partially supplied by the NNE/SSW trending Davagh esker complex, which conducted subglacial meltwater and debris northwards from the Ballinderry valley lowlands, against the local topographic gradient. | |
| Description: | |
The deposits of the Lough Fea complex are categorised on the basis of their morphology (ridges, terraces and plains) and sedimentology, where exposed. The complex comprises seventeen subunits covering the Lough Fea platform and the flanks of the upper Moyola valley. Sediments exposed consisted of broadly similar lithofacies associations (LFAs) and sedimentary structures. Two exposures illustrating the important sedimentary architectures and grain size characteristics for the interpretation of the complex's palaeoenvironmental conditions are described in detail. 1. Crockandun Terrace A small-scale (0.125km2), flat-topped terrace located between 310 and 320m O.D. on the southeastern flank of Glenarudda mountain (528m O.D.) on an eastern margin of the Slieve Gallion massif. The peat-covered flat-top which has been extensively quarried is bordered on the northeast by a steep slope and includes two small-scale, water-filled enclosed depressions. 2. Corick Terrace A relatively extensive (0.5km2) flat-topped terrace at 280m O.D. located at the base of the steep northeastern slopes of the Crockandun terrace. An exposure in a partially landscaped, large-scale commercial quarry consists of poorly-sorted, planar cross-bedded sands and cobble gravels dipping at 20§ to 030§N. The flat-top is bordered by a steep northwestern slope and low gradient (<10§) northeastern flank. 3. Carnabane Ridge and terrace A steep sided, 0.25km long by 0.1km wide sharp-crested, northeast/southwest trending ridge occurs at 250m O.D., northwest of the Corick terrace. An exposure in the eastern end of the ridge, where it connects with the 0.188km2 flat-topped Carnabane terrace, consists of well-sorted sands, clast-supported cobble gravels and very poorly sorted boulder gravels in a variable granule sand matrix. The beds are contained in channels and dip at low angles (<5§) generally to 040§N, grading in this direction into the terrace. The sand beds in the terrace tend to increase in thickness to the northeast, parallel to the orientation of the ridge. Clast a-axes range from 5cm to over 2 m. Clasts are predominately (>70%) granitic, with smaller amounts of limestone and basic igneous rocks. 4. Brackagh Ridge and Plain At Brackagh an extensive (1.25km2) area of low relief is punctuated by a discontinuous <0.5km long sharp-crested ridge at the northern corner of the Lough Fea platform. The surrounding flat surface (at 240m O.D.) is overlain by a thick (>1.5 m) predominately intact peat mantle containing several enclosed, water filled depressions. A large-scale commercial aggregate quarry (approximately 0.005km2) exposes stratified sediments in a 150m long by 20 high ridge segment and the southern edge of the flat plain. The plain, exposed in the northern face of the quarry, consists of rippled and horizontally laminated well sorted medium to pebbly sands occurring in broad (>30m), shallow (<2m deep) channels. Palaeoflows, shown by ripple foreset orientations and channel axes are to the north and northeast. The sediments exposed in the southern face of the quarry consist of poorly-sorted, chaotically bedded cobble gravels interbedded with cross-bedded sands. Palaeoflows, as indicated by small-scale cross-beds and channel axes, are highly variable over short distances but generally trend to the north and northeast. 5. Slaught Ridge A 0.5km long, by 0.05km wide, by 10m high, 0.075km2 ridge occurs at 240m O.D. at the northern margin of Fir Mountain. It is oriented NE/SW, at right angles to the axis of the depression containing Lough Fea. The northwest facing slope is up to 15§ and a small-scale exposure consists of sheared and disturbed sands and cobble gravels. The stacked shear planes strike sub-parallel to the ridge axis, and rise to the northeast. 6. Stone Hill Ridge An elongate, streamlined 0.088km2 ridge segment with a local relief of 25-30m occurs at 274200 389300 in the Brackagh plain. The long axis of the segment is oriented approximately northeast/southwest. Clusters of large boulders, with a-axes lengths of 1m occur on the ridge flanks. 7. Mill Lough Ridge A chain of ridge segments occurs along the northeastern rim of the Teal Lough plain in an arcuate band approximately 1.5km long and 0.25km wide occupying 0.31km2. An exposure at Mill Lough consists of channelised, small-scale planar cross-bedded sands with occasional cobble clasts, horizontally bedded pebble gravels and crudely-stratified cobble gravels. Palaeoflow measurements, based on channel axes, ripple laminations and small-scale cross beds, are generally to the north and northeast. 8. Teal Lough Plain A 1.39km2 area of low local relief extends from the western slopes of the Slaught ridge westward and northward to the margins of the platform. The margins are formed by the rising slopes of Fir Mountain to the south, the Moyola Valley to the north and northwest, and the northern end of the Davagh ridge complex to the west. The bog covered area contains enclosed depressions occupied by bodies of standing water. No exposures occur in the plain. 9. Davagh ridge complex Located along the western margin of Beleevnamore mountain, a steep sided, discontinuous ridge complex, composed of narrow (<30m across) sharp-crested and broader, flat-topped sections occurs to a maximum elevation of 210m O.D. at its northern end. This end of the complex consists of a 0.75km long hummocky arcuate ridge of 15m local relief oriented northwest/southeast located at the western edge of the Teal Lough plain. The complex of branching ridges continues downslope for 5km with a <0.5km-long break at Black Rock (268800 383400) at approximately the same local relief southwestward along the foothills of Beleevnamore Mountain to 150m O.D. near Dunnamore in the Ballinderry River valley (see Key Site 480 - Ballinderry). One small-scale exposure (10m long by 15m deep) in the flank of the ridge at 268300 381200 consists of large scale (<20m deep) planar cross-beds of clast supported boulder gravels and massive boulder gravels. Poorly defined a-axis imbrication of the clasts (50-60cm) and cross-bed dips (20§) indicate a palaeoflow to 360§. 10. Black Lough Ridge A locally topographically prominent, northeast/southwest oriented, steep-sided 0.188km2 cross-valley ridge is located at 276600 487800, at the northern end of the valley containing the Lough Fea depression. The ridge is composed of interbedded well sorted sands and cobble gravels in channels and tabular beds of massive, poorly sorted boulder-sand admixtures. Palaeoflows, based on channel axis orientations and bedding dips, are to the north and northeast. 11. Ballybriest Ridge A topographically prominent, dissected, 0.68km2 ridge with remnants of a flat-top occurring in places (at 253m O.D.) occurs along the axis of the lowlands between Slieve Gallion and Fir Mountain. Extensive (commercial quarry) exposures of sediments show the ridge to be composed of massive boulder gravels (>4m exposed to base of section) overlain by over 25m of clast-supported beds (>0.5m thick) of large-scale planar cross-bedded cobble gravels with occasional discontinuous sand and silty sand beds. Sands are ripple laminated and the pebble gravels parallel bedded and planar cross-bedded. On the western margin of the ridge, bordering the Lough Fea depression, faulted and folded trough-bedded cobble gravel beds, with fault downthrows to the west, are also exposed. In the southern part of the exposure, the cobble gravels are overlain by 5m of laminated silt and sand. 12. Lough Fea Depressions Between the western edge of the Ballybriest ridge and a small (0.03km2) topographic flat at 275300 386400 the water body of Lough Fea occupies a 0.3km2 enclosed depression. Smaller (<0.01km2) depressions are located 0.5km further east, in the Ballybriest ridge (the Black Loughs) and further south at the western edge of the Grouse Lodge ridge (containing The Fly Lough). 13. Grouse Lodge Ridge A dissected, undulating 0.2km2 cross-valley ridge with flat-topped segments is located immediately south of the Ballybriest ridge and Lough Fea depressions occurring between 230 and 250m O.D. The ridge consists of massive beds of coarse grained diamict, small-scale planar cross-bedded and planar bedded sands and gravels, overlain by low angle (<10§) large-scale planar cross-bedded sand and silty-sand. Palaeoflows were to the north and northeast. Dardis (1982) documented a 5m sequence of sand and unusually thick (10-20cm) clay beds at Grouse Lodge (277000 385600), 0.5km due east from the exposure documented above. Similar couplets occur at Ballybriest, 1km further north. 14. Lissan Ridges A series of small ridges (0.16km2) cross the upper Lissan river valley. They range in altitude from 190m O.D. on the valley sides to 120m O.D. in the valley centre. Dardis (1982) documented an exposure in one ridge at Dunnamore (278400 384200) which consisted of diamicts and disorganised boulder gravels containing laminated clay drapes, overlain by planar cross-bedded boulder gravels and finally silt and sand couplets. 15. Moyola Valley and Disert Terraces In the upper Moyola river valley, sets of dissected flat topped terraces occur on both sides of the river between 271000 389900 and 276600 390800 at two discrete altitudinal levels (150 and 120m O.D.). These are found on the lower northern slopes of the escarpment forming the Lough Fea platform at the mouth of a large rock-cut channel. The altitude of the flat-tops (0.325km2) correspond in altitude with the valley-fill terraces which occur at Disert (276400 492600) 5km downvalley. There, a pristine small-scale (<1km2) area consisting of a flat-topped, dissected valley-fill occupies the lower portion of the White Water river valley. Dissection has divided the deposit into two unequally sized terraces occurring at the same elevation (150m O.D.) on opposite sides of the valley. No exposures are present. | |
| Importance: | |
The Lough Fea complex is rated as highly scientific important due its demonstration of: 1) A wide variety of deglacial depositional environments (outwash plain, morainic ridges, fan deltas, subaqueous moraines) and sediment supply mechanisms (e.g. eskers, meltwater channels, sub-aqueous effluxes) 2) The coincidence and juxtaposition of glacigenic landscape elements (e.g. esker orientation and moraine ridge position; moraine ridges and outwash plains), showing the spatial and temporal evolution of the complete deglacial landform development sequence from sediment supply routes through to subaerial reworking and erosion of sediments 3) The influence of bedrock topography upon the spatial pattern of deglaciation, ice mass splitting and lobe formation and the location of ice marginal deposits 4) The formation of kettle holes, namely the depressions occupied by Lough Fea, Teal Lough, Mill Lough, Black Lough, Cow Lough and Lough Patrick. 5) The southerly origin of ice which occupied this region of the Sperrin uplands during the last glacial cycle. | |
| Interpretation: | |
The landform and sediment assemblages are inferred to represent ice-marginal deposition during from the phased deglaciation of the Lough Fea platform. It is inferred from both the local granitic and basic igneous erratic content of the deposits and the evidence of ice movement patterns from adjacent areas that the area was occupied by ice of southerly origin during the last glacial cycle. During the deglacial cycle, ice on the Lough Fea platform separated from Bann valley ice occupying the Moyola Valley lowlands. It then progressively downwasted and backwasted generally southwards across the platform, due to ice pressure from two lobes, one located in the Broughderg valley to the southwest and the other in the Lissan valley to the southeast. The pattern of ice retreat was thus controlled by the location of local bedrock highs and the direction of ice pressure. The ice finally retreated into the lowland areas south and east of the Sperrin Mountains, accompanied by production of ice marginal and proglacial lakes due to the impounding of meltwater against the rising ground of the mountain range (cf. Dardis, 1985). The sequence of landform formation is interpreted to be as follows: 1) During deglaciation, an ice mass of southwesterly origin covered the area to at least the elevation of the terrace at Crockandun Hills (320m O.D.) (Site 1). The ice mass had probably overtopped the peaks of Fir mountain, Oughtmore and Beleevnamore to the southwest of the platform carrying in large amounts of Tyrone igneous granite and basic igneous rocks. 2) The terrace at Crockandun Hills formed by deposition of sediments in a small waterbody impounded in an ice marginal location against the rising ground of the Slieve Gallion massif. The steep, northwestern slopes of the terrace and the orientation of distributary channels in the sediments indicate that the main ice lobe was located on low ground of the platform to the northwest and west. 3) As the ice back- and downwasted, a fan delta formed in an ice marginal ponded waterbody with a surface elevation of 280m O.D. giving rise to a second, lower, flat-topped terrace at Corick (2). 4) Continued sediment supply to the downwasting ice margin resulted in the formation of a subaqueous fan at Carnabane (3), between 250 - 260m O.D. followed by a lowering of the ice-marginal waterbody surface. A sharp-crested ridge segment adjoining the flat-topped terrace could represent a subglacial, confined, high energy sediment supply route. 5) At the northern margin of the platform, the sands and gravels at Brackagh (4) indicate low energy sedimentation within broad, shallow channels, interpreted to be part of a low-angle, subaerial outwash fan. Ridge segments and coarse-grained gravel at the southern and northwestern periphery of the plain indicate the position of the ice-marginal efflux point during the deposition of the adjacent outwash plains. 6) The ridge segment alignments indicate that the ice margin was progressively contracting and withdrawing locally southwestwards. Further retreat is marked by similar coincidences of isolated ridge segments and areas of outwash e.g. Teal Lough plain (8) and Davagh esker ridge (9). The ridges can be traced in arcuate bands across the outwash plain, possibly representing ice-pushed morainic ridges. 7) The kettling of the outwash plains indicates the rapid aggradation of sediments and burial of ice blocks around a stagnating ice margin. The kettle hole locations form a generally northeast/southwest trend, inferred to reflect ice margin position as retreat progressed southwestwards. 8) During the development of the outwash plain adjacent to Stone hill ridge (6), the ice mass is inferred to have been increasingly subject to pressures exerted by bedrock topography, subsequently splitting into two separate lobes, along a north/south line sub-parallel to the northeastern spur of Fir Mt. The ridge at Slaught road (5) marks a lateral moraine of the westward retreating (Davagh) lobe and the ridge segments at Mill Lough (7) a later position of the ice margin. 9) The sediment supply route to the western portion of the platform is marked by the linear Davagh ridge complex (9) to the west of the rising bedrock topography of Beleevnamore mountain. The ridges are interpreted to be an esker complex formed within a subglacial conduit. Sediment laden meltwater was pumped northwards and uphill (60m over 5km) under high hydraulic pressures. The system may have supplied the majority of sediment forming the western Lough Fea deposits. Elements of the esker ridge may be buried at depth under the later outwash sediment sequences now forming the surface topography of the platform. 10) Such a sediment sequence was observed at Ballybriest, with a core of massive gravels overlain by temporally later, lower energy, glaciolacustrine sands and gravels. This may indicate the sediment supply route for deposits formed from ice which backwasted southeastwards along the Lissan valley. 11) The southward retreating (Lissan valley) lobe formed the ridge at Black Lough through the development of a series of proglacial fans, with generally north and northeastward dipping mass-flows and sands exposed in the ridge sediments, and steep, possibly ice contact slopes forming the southern flank. 12) Stagnation of a section of the ice-lobe margin caused the burial of relatively large ice blocks in sediments prograding from the south. This 'dead ice' and the ridge at Black Lough formed temporary meltwater dams causing the impoundment of water in the basin between Fir Mt. and Slieve Gallion. The Ballybriest and Grouse lodge ridges represent the proglacial glaciofluvial deposition of coarse-grained fan deltas into these localised waterbodies. The lowering of bedrock topography southwards in the Lissan river valley and the further retreat of the margin allowed the development of thick rhythmite sequences in a deeper lake located in the Grouse Lodge area (Dardis, 1982). 13) The melting of the buried ice blocks gave rise to topographic inversion, depressions now occupied by Lough Fea, the Black Loughs and The Fly Lough. 14) Ice located in the Lissan valley formed a series of cross-valley ridges interpreted to represent sublacustrine moraine deposition (Barnett and Holdsworth, 1974) formed from sublacustrine sediment effluxes. 15) Subaerial erosion of the highly erodible, unconsolidated sediments by meltwater resulted in the formation of deep meltwater channels (now occupied by the Black Water and White Water rivers) and valley fill terracing at Disert and the upper Moyola valley, both formed in a waterbody ponded at 150m O.D. in the Moyola Valley (see Key Site 476 - Moyola). Preferential erosion of finer sediments resulted in the isolation of boulder-sized clasts on the present day surface. Two authors have on previous occasions commented on the deglaciation of the Lough Fea platform. A brief synopsis of their models and the differences between them and the interpretation proposed in this report follows: Colhoun, E.A. (1971) The deglaciation of the Sperrin Mountains and adjacent areas, Section 13. The Slieve Gallion Ridge and Lough Fea Platform. Colhoun traces the phased retreat of a single ice lobe generally southwards across the Lough Fea platform after separation from eastward retreating ice in the Moyola Valley. The pattern of ice retreat is recorded through ten distinct episodes of moraine and outwash terrace deposition, using strictly morphological criteria to identify the deposits. The pattern of meltwater flow and ice-marginal position indicators needed to comply with this model are not fully supported by the evidence presented here. Bedrock topography and the envisaged sediment supply route of the Davagh esker result in the recorded palaeoflow patterns of meltwater drainage and ice-marginal positions. Dardis, G.F. (1982) Sedimentological Aspects of the Quaternary Geology of south-Central Ulster, Northern Ireland, Unpublished PhD Thesis, Univ. of Ulster. Dardis (1982) classified cross-valley ridges on the basis of their (1) external morphology and sedimentology, (2) position with respect to ice-contact slopes and (3) associated landform elements. The deposits of the Lough Fea platform were seen to provide examples of each of the four ridge types, deposited over time in extensive proglacial ponded waterbodies impounded against the Sperrin uplands and the margins of the ice as it retreated southward into the Lough Neagh Lowlands. Outlets, located further to the north, resulted in ten major regional glacial lake stages in south-central Ulster with levels controlled by outlets located to the north Several of the deposits documented in this report are not included in the Dardis' model whilst others occur at elevations not consistent with the elevations of the extensive lakes in the model (i.e. they occur above the highest lake elevation of 250-255m O.D.). Glaciolacustrine deposition in smaller-scale ponded waterbodies of more restricted areal extent is instead envisaged to have occurred in ice marginal locations. | |
| 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 This site is also a Site of Local Nature Conservation Importance. See Key Site 737 - Lough Fea. | |
| Keywords | |
|---|---|
| Rocks: | Gravel, Sand |
| Relations: | Glacio-fluvial sediment, Glaciolacustrine deposit |
| Geomorph: | esker, kame terrace, kettle-hole, moraine ridge, raised bog |
| Measurements | |||||
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| Length: | Width: | Height: | |||
| Depth: | Area: | 33km2 | |||
| Access | |
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| Management: | |
The area contains a wide variety of topographic elements including flats, ridges, hummocks and kettle-holes, as yet largely unobstructed by conifer plantations. The concentration of large-scale commercial quarrying operations in the deposits, however, dominates the conservation issues and assessment of the aesthetic condition of the area. The commercial pits, the processing machinery and pit spoil-heaps starkly contrast with the heather covered mires of the platform and rising slopes of the surrounding uplands detracting from the excellent vistas of the southwestern margin of the Sperrin Mountains range occurring in the area from the deposits. Commercial aggregate production, has by-products; an increase in the amount of heavy machinery and vehicles on otherwise peaceful country roads, increased noise pollution, as well as the production of unsightly and potentially hazardous spoil. At the Black Loughs it is clear that the freshwater ecosystems existing in the water-filled kettle holes are at present under threat from the encroachment of this often silt-laden spoil. The area provides an excellent example of close relationship between raised bog and glacigenic topography, notably at Brackagh where a thick (>1.5m) extensive (>1km2) relatively pristine raised bog has formed in association with the poor drainage of the flat outwash plain. However, the association of such ecosystems with occurrences of such valuable mineral aggregate deposits also places them under the threat of removal to facilitate the further expansion of the quarries. | |
| Uses | |
| Uses: | Large-scale commercial quarrying |
| References (3) | |||
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| Barnett, D.M. & Holdsworth, G., 1974. Origin, morphology and chronology of sublacuatrine moraines, Generator Lake, Baffin Island, Northwest Territories, Canada Canadian Journal of Earth Science, vol. 11, pp.380-408. | |||
| 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. | |||
| Dardis, G.F., 1982. Sedimentological aspects of the Quaternary Geology of South-Central Ulster, Northern Ireland (Thesis), pp.422 Unpublished PhD Thesis, New University of Ulster. | |||
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Lough Fea, County Londonderry, Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=477. Accessed on 2026-11-06. |
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