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Earth Science Conservation Review
| Clogher Valley | Tyrone |
| Site number: | 486 | ||
| ESCC: | IS | ||
| Locality Type: | Inland exposure | Status: | TBC |
| Grid Reference: | H630575,H386406,H372330 Corners of site, Not Entered | ||
| Google maps: | 54.46254,-7.02847 | ||
| County: | Tyrone | Council area: | Fermanagh & Omagh District Council |
| Period: | Quaternary | ||
| Site Description | |
|---|---|
| Highlights: | |
The Clogher Valley complex records ice marginal and subglacial sedimentation of glaciofluvial and diamict facies during glaciation and southwesterly ice retreat into the lowlands of the central Clogher Valley and the Lough Erne Basin from the surrounding uplands. The Clogher Valley is dominated by a discontinuous system of esker-like ridges trending northeast-southwest, and ice marginal moraines. An exposure at Drumhirk (H547544) records local proglacial sedimentation of massive, stratified and rippled sand and silt against the south side of the Fintona Hills. | |
| Description: | |
The Clogher Valley complex is underlain by limestones and shales of Tournaisian (Carboniferous) age. These form a fault-bounded wedge in the central part of the valley with Old Red Sandstone (ORS) and Carboniferous basal clastics to the north and Carboniferous sandstones to the south (Wilson, 1972). These rocks are overlain by a drift cover of variable thickness (10-30m). The eastern side of the Clogher Valley complex is defined by the Fintona Hills to the north and the foothills of Slieve Beagh to the south. The western side is defined as the limit of eskers and associated glaciofluvial sediments. The complex extends from Ballygawley in the northeast (H630575) to Brookeborough (H386406) and southeast of Lisnaskea (H372330), a total of 36km long. Sand and gravel is generally confined to a belt 3-4km wide along the valley axis. The Clogher Valley complex can be divided into 4 main areas based on morphology, sediment type and sediment ridge orientation. There is very little exposure, hence a reliance on secondary data sources and morphology to establish the characteristics and direction of ice retreat. These areas are as follows: (1) The eastern area between Ballygawley and Augher This area, about 7km long, is characterised by a series of elongate ridges, averaging 600m long, 200m wide and 20-30m high. These ridges, composed of diamict, are highly variable in orientation and morphology. They are composite, with localised and isolated hills, or are broken by meltwater incision. These hills often form vague 'rings' or 'corridors' 1-1.5km in extent within which are flat areas of peatland. These peatlands also characterise the areas between individual ridges. There are no sedimentary exposures. Ridges are interpreted as ice marginal moraines recording south and southwestern ice retreat along the valley axis after the regional ice cover fractured over the axis of the Fintona Hills to the north. Local enclosed peatlands formed as a response to water build-up or non-drainage associated with two main processes: (1) ice block detachment during marginal stagnation. The 'ringing' of these peatland areas, such as at Ballylagan (H594590) and Derrynascobe (H577518), by diamict hills, indicates that sediment throughput associated with block detachment was high, and probably with a high water content. Diamict deposition was likely to have been from mass flow processes, some at least of which must have been subaerial. (2) Distinct phases of ice recession, with discrete marginal moraines up to 1.2km long, infer periods of dynamic sediment conveyance to the margin interspersed with periods of local meltwater production and possible moraine breaching during marginal stillstands. Subglacial material may have been thrust up into moraines by oscillations of the ice margin. Ice movement from the Fintona Hills was accompanied by the build-up of local sand and gravel outwash surfaces at 120-130m as at Ballymackilroy (H604540), Ballynasaggart (H598572) and Kilgreen (H585571). Dardis (1982) explained the Ballymackilroy system as fed by a small esker trending south-north, although the present morphological evidence for this feature is equivocal. It is more likely that all these sand and gravel spreads are of a similar age, and relate to regionally-extensive ice withdrawal and the impounding of local water bodies between the retreating ice front and the bedrock uplands. (2) The area between Augher and Mullaghmore This area can be divided into two sectors oriented along the axis of the valley. This division is made with reference to substrate type and morphology. Sector A fronts the south side of the Fintona Hills, is 1-2km wide and composed mainly of isolated sand and gravel hills. Sector B fronts the north side of the Slieve Beagh foothills, is 2-3km wide and composed of diamict ridges. SECTOR A This sector is characterised by an undulatory topography of isolated, flat-topped hills up to 400m long, 200m wide and 20m high dominated by glaciofluvial sediments. In low ground areas (<100m), these hills have meltwater oversteepened slopes, and are separated by present river floodplains with alluvial fills. Peatlands do not generally occur. In higher ground areas (>100m), discontinuous flat surfaces onlap onto both bedrock and diamict, with the drift limit highly variable. Diamict generally reaches 180m; lowland sand and gravel to about 90m. An exposure at Drumhirk (H547544) near Augher is the only significant exposure in the entire Clogher Valley. The Drumhirk pit, 2km northwest of Augher, is a largely disused sand and gravel pit covering c. 2ha. Approximately 30% of the pit offers exposure, mainly in the northwestern area where sporadic workings show faces 3-5m high. Facies description: This exposure is characterised by stacked and interbedded rippled and massive sand, occasional boulder lags and scour and fill structures. Other features shown are well-developed sand sheets, massive silt and clay drapes, sand wedges and high angle normal and reverse faults. Directional indicators based on climbing ripple direction and cross-bedded units are highly variable, but show general water flow towards 030-075° and 200-220°. In more detail, massive and stratified medium- and fine-grained sand facies (0.2-2m thick) are interbedded with massive matrix- and clast-supported gravel (0.6-1.5m thick and laterally discontinuous) with both channelised erosional and graded bed contacts. These facies are conformably overlain by a discontinuous sandy diamict <1.5m thick. Sand units are generally well-sorted and undisturbed by glaciotectonics. Facies interpretation: These sediment sequences are characteristic of proglacial subaqueous deposition in a local impounded water body. Highly variable interbedded facies of well-sorted sediments indicate periodic sediment pulses, probably by resedimentation and mass flow processes. Further, lack of glaciotectonic deformation, lack of prominent erosional surfaces, and dominance of fine grained sediments, indicates that there was little energy available in the system. However, lack of clay drapes, and continuous sand accretion, also indicates that this energy/sediment input was maintained and varied little (i.e. not subject to seasonal rhythmicity). Local flow to 200-220° may indicate that periodic meltwater input was also derived from the Fintona Hills, suggesting that ice had not withdrawn very far northward into the Omagh Basin at this time. As ice withdrew southward in the Clogher Valley, this impounded lake was probably drained by ice-marginal channels flowing parallel to the valley axis. Drainage was followed by a period of subaerial exposure and a severe climate. This gave rise to the sand wedge, which is stratigraphically below the resedimented diamict overlying much of the exposure. This facies may have been emplaced by mass flows from the adjacent high ground and records greater water availability and sediment instability during climate amelioration. SECTOR B This sector is characterised by large (1.2km long, 200m wide, 30m high) composite diamict ridges, generally oriented along the valley axis, i.e. approximately NE-SW (Charlesworth, 1924). Around Clogher (H541520) these ridges are highly diverse in morphology with generally steep slopes, and are associated with meltwater channelling and local inter- and intra-ridge areas of flat, Carex-dominated wetland. These ridges are very different in overall morphology, orientation and setting to the recessional cross valley-type moraine ridges further up and down valley. There are no sediment exposures in this area. Sector B is interpreted to be composed of composite moraines representing periods of changing ice activity associated with general south and southwestward ice marginal retreat. These moraines reflect the conveyance of material to the ice margin as the ice changed configuration, with (probably) high facies variability and thrust sheets. If this interpretation is accurate, then the development of 'stagnation-type' topography from ice block inclusions within the marginal morainic sediments may not an accurate descriptor of true ice-marginal dynamics (Kurimo, 1980). Change in moraine morphology may also be related to lake impoundment to the north (sector A). Moraines to the northeast of Knockmany Hill (area 1) are generally small, interlocking and oriented NW-SE. Moraines 1km south of Knockmany Hill (area 2, sector B) are large, isolated and aligned NE-SW. Knockmany Hill acted to disrupt ice backwasting and constrain lake expansion along the south side of the Fintona Hills. This relationship also indicates that sector B is of a later age than sector A. (3) The area between Mullaghmore and Brookeborough This area is characterised by linear sand and gravel esker ridges aligned NE-SW along the axis of the valley, with associated spreads of glaciofluvial sediment extending 0.5-1.5km on either side. This sediment forms a distinct area 12km long and 1-2km wide, beyond which are diamict ridges aligned northwest-southeast. Therefore this sand and gravel area is distinct in terms of both ridge alignment and sediment type. Individual esker ridges are up to 20m high and 2km long. They are generally steep-sided, bifurcated both in an up- and downvalley direction, with fairly flat tops. Ridge relationships include tributary and distributary eskers, wide areas of eskers aligned parallel with one another, and breaks between esker ridges on the scale of 5-100m. Also, esker ridges may be superimposed over the top of diamict ridges. Glaciofluvial sediment spreads onlap or is smeared onto the adjacent diamict moraines. A small exposure around Fivemiletown (H459482) in the associated flanking sediments shows massive, medium-grained sand, typical of glaciofluvial outwash. No other exposures are recorded, and there are therefore no conclusive indicators of flow direction along this esker. High-density boulder clasts typical of subglacial conduits were once exposed at Andrew's Wood (H488490) (A.M. McCabe pers. comm.). Charlesworth (1924, p.271) records 'water-worn pebbles, sand and gravel' at Ballyraddan (or Ballyvaddan) (H456483). These clasts 'include quartzite, carboniferous limestone, sandstone, shale, conglomerate, an occasional [quartz-] porphyry and epidiorite' (Charlesworth, 1924, p.271 Note 1). Igneous and metamorphic clasts are derived from the Lough Derg and Baronscourt areas to the north, and the Tyrone Igneous Complex to the northeast (cf. McCabe et al., 1978). Clasts of sedimentary lithology are locally derived. Additionally, McCabe et al. (1978) record ORS carriage to the Fivemiletown area from Brougher Mountain, 15km to the west. Therefore (a) assemblage provenance is not an effective indicator of ice flow direction, and (b) clasts appear to be reworked from previous deposits. From evidence of ridge morphology and direction, and secondary data on sedimentary characteristics, this landform assemblage is interpreted to represent subglacial esker deposition, probably trending northeast to southwest, exposed during later periods of both ice retreat and stagnation. Complex bifurcated ridges, beads and fans of associated sediment, and areas of non-deposition, favour periodic sedimentation in water-filled subglacial cavities, with changes in flow energy, sediment grade and sorting over time (Gorrell and Shaw 1991). This may also involve sediment reworking along the subglacial conduit as meltwater was expelled from the system, and possibly the later deposition of (proglacial?) sandy sediments that onlap the adjacent diamict ridges. (4) The area between Brookeborough and Lisnaskea This area is characterised by discontinuous mounds of sand and gravel, interspersed with areas of exposed bedrock and drumlins. Meltwater channels are also present, and are up to 15m deep. Sand and gravel generally forms a broken ridge, usually between the 80-90m contours. Southwest of Brookeborough, these sediments are juxtaposed with a prominent bedrock ridge, conformable to the sand and gravel ridge axis, which trends northeast-southwest. This conformity is probably due to (1) sand and gravel sedimentation in a pre-existing bedrock cavity or low, and (2) later meltwater dissection that has given a uniform shape to both sand and gravel and bedrock ridges. Presence of the bedrock probably helped to channelise meltwater, encouraging downflow dissection. Between Brookeborough and Lisnaskea, sand and gravel is more discontinuous, often overlying bedrock to the east of the complex, at about 100m elevation. Around Hollybrook (H329374) there is a small (250m wide) fan fed by conduits that also supported the formation of the sand and gravel ridges (eskers) further upvalley. This fan indicates that an ice margin must have been present across the neck of the Clogher Valley, fed by water flow trending northeast-southwest. At this location, sand and gravel lies superimposed over thin diamict and cold-climate solifluction debris (McCabe, 1969a). Between Brookeborough and Lisbellaw isolated diamict drumlins dominate. This contrasts with areas further upvalley where moraines, and diamict ridges of composite form, dominate. These drumlins are characterised by an upper drumlinized diamict below which are gravels and organic silts which overlie a lower diamict (McCabe et al., 1978). Both diamicts are dominated by northern and local clasts (mainly Carboniferous limestone, sandstone and ORS) (McCabe, 1969b). Two main characteristics differentiate deposits in the Brookeborough-Lisnaskea area from those further upvalley, such as around Ballygawley. These are: (1) Material of a much older relative-age is preserved and/or exposed downvalley. Exposure of this older material is facilitated by a thin overlying (later) diamict mantle. (2) Sand and gravel downvalley is much more distinct in morphology and extent. Around Ballygawley, sand and gravel lies in intimate association with diamict ridges, possibly because of active ice-marginal processes such as glaciotectonism. Downvalley, sand and gravel is confined and superimposed upon both underlying diamict and bedrock. Therefore the processes controlling sand and gravel deposition in this southern area is very different in time and characteristics from those controlling sand and gravel deposition around Ballygawley. | |
| Importance: | |
The Clogher Valley complex is of rare importance by virtue of its location, extent and morphological characteristics. This is because: A) It is important in linking the glacial systems of central Tyrone/Sperrins and the north Midlands. Very little is known about the latter. B) Moraine ridges around Clogher are unique in morphology and inferred composition, and relationship to surrounding features. Similar topography has not been described elsewhere. C) Subglacial (Rogen-type) moraine ridges have not been formally described from Ireland before. D) Esker development reflects a regional pattern of subglacial conduit drainage around the Omagh Basin. E) Evidence for (i) local sand and gravel deposition against the Fintona Hills, (ii) meltwater flow reversal, and (iii) cold climate conditions after deglaciation are in agreement evidence from other areas (McCabe, 1969a; Colhoun, 1971). | |
| Interpretation: | |
Charlesworth (1924, p.270-1) believed that Clogher Valley ice retreated westwards along the valley, based on interpretations of lateral moraines along the valley sides, especially along Slieve Beagh to the south. Further, he interpreted the sand and gravel ridges at Fivemiletown (H445479) and Colebrook (H415444) as part of composite ice-marginal moraine and outwash features associated with 'countless lakes' (p.270). McCabe (1985, his Fig. 6C, p.80) interprets the same ridges as (subglacial?) eskers superimposed upon a 'drumlin and interdrumlin hollow' landscape. These eskers would then appear to feed southwards in association with small ice-contact and ice-pushed moraines during marginal retreat to the northeast (McCabe, 1969b). The sand wedge seen in the Drumhirk exposure is in line with indicators elsewhere for a regionally-extensive period of cold climate following ice withdrawal (McCabe, 1969a, Colhoun, 1971, McCabe et al., 1978). Sand and gravel (subglacial esker) ridges are usually superimposed upon, and are different in directional alignment, to diamict ridges. If a subglacial origin is inferred for the sand and gravel ridges, with deposition following the hydraulic gradient to the ice margin (Shreve, 1985), the primary morphology and alignment of diamict ridges infers formation in a subglacial environment transverse to (northeast-southwest) ice flow direction. Transverse subglacial moraines (Rogen moraines) formed of diamict have been extensively described from Scandinavia (i.e. Lundqvist, 1989), not at all from Ireland. Rogen moraines may be associated with a range of ice velocities and subglacial environments, but is probably a transitionary bedform that may grade both over time and downglacier to drumlin bedforms (Lundqvist, 1989). A general requirement (like drumlins) is for thin ice and fast ice flow. Therefore Rogen deposition may not be greatly older than the overlying eskers, especially given that both require water availability for subglacial deformation. Moraine ridges in the Clogher Valley are almost perpendicular to drumlin and striae directions (Charlesworth, 1924). Evidence hindering an equivocal landscape interpretation includes: (1) Lack of sedimentary exposure. Sediment types, structures and provenance are largely unknown. (2) Over-reliance on morphological evidence, mainly due to (1). (3) Lack of definitive stagnation topography. This would indicate where the last ice disintegrated, whether upvalley or in the Lough Erne Basin. (4) Extensive dissection by meltwater. 'Ridges' may not always be primary features. (5) Shallow valley gradient. Water could have flowed from any direction, especially if under pressure. The following sequence of events in the Clogher Valley is suggested on the basis of the geomorphic and sedimentary evidence outlined above: (1) Late Devensian ice flow north to south across the Fintona Hills and associated diamict deposition (McCabe, 1969b). Multiple striae sets on Slieve Beagh record changes in Late Devensian ice flow direction, possibly related to ice dome migration. (2) Rogen moraine formation in a subglacial environment associated with northeast-southwest ice flow. (3) Subglacial esker deposition, probably northeast-southwest (but there are no equivocal indicators of flow direction apart from drumlin and striae alignment (Charlesworth, 1924)). (4) Ice downwasting over the Fintona Hills and a change in hydraulic gradient leading to a reversal in meltwater flow direction, from northeast-southwest (cf. esker alignment) to west-east (sand and gravel deposition at Kilgreen). (5) Ice margin retreat east-west; moraine emplacement at Clogher. (6) Change in ice marginal configuration to the north of Slieve Beagh. Deposition of Drumhirk sands in an impounded lake. Incision by meltwater drainage. (7) Further south and southwest ice retreat of the northern margin. Retreat north of the southern margin with meltwater incision and probable ice stagnation around Brookeborough and Maguiresbridge. (8) Postglacial alluviation and raised bog development. | |
| 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 | |
|---|---|
| Rocks: | Gravel, Sand |
| Relations: | Glacial deposit, Glacio-fluvial sediment, Ice retreat, Ice stagnation |
| Geomorph: | esker, moraine ridge |
| Access | |
|---|---|
| Management: | |
Few aggregate pits in this area infer generally high morphological intactness. It may also indicate the dominance of fine-grained sediments, and that any gravels present have been already extracted (i.e. at Ballyraddan). Specific aesthetic attributes include: 1) The physiographic distinction of the Clogher Valley as a whole. This means that an overall planning/management policy can address valley-wide issues. 2) Geomorphic intactness, especially of the esker ridges, such as around Colebrook (H415444). 3) The juxtaposition of sediments and depositional environments, such as esker ridges and moraines between Brookeborough and Fivemiletown which lie perpendicular to one another. These may represent the superimposition of eskers over Rogen moraine. 4) Lowland alluviation and floodplain development associated with the Colebrook River. These flat areas contrast with both eskers and the bedrock-dominated slopes of Slieve Beagh. |
| References (10) | |||
|---|---|---|---|
| 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., 1971. Late Weichselian periglacial phenomena of the Sperrin Mountains, Northern Ireland Proceedings of the Royal Irish Academy, vol. 71 pt. B, pp.53-71. | |||
| 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. | |||
| Kurimo, H., 1980. Depositional deglacial forms as indicators of different glacial and glaciomarginal en. | |||
| Lundqvist, J., 1989. Rogen (ribbed) moraine - identification and possible origin Sedimentary Geology, vol. 62, pp.281-292. | |||
| McCabe, A.M., Mitchell, G.F. & Shotton, F.W., 1978. An inter-till freshwater deposit at Hollymount, Maguiresbridge, Co. Fermanagh. Proceedings of the Royal Irish Academy, vol. 78, pp.77-89. | |||
| McCabe, A.M., 1969a. The glacial deposits of the Maguiresbridge area, County Fermanagh, Northern Ireland Irish Geography, vol. 6, pp.63-77. | |||
| McCabe, A.M., 1969b. A buried head deposit near Lisnaskea, Co. Fermanagh, Northern Ireland Irish Naturalists Journal, vol. 16, pp.232-233. | |||
| McCabe, A.M., 1985. Glacial Geomorphology In The Quaternary History of Ireland, pp.67-93 Academic Press, London. | |||
| Wilson, H.E., 1972. Regional Geology of Northern Ireland. HMSO, Belfast. | |||
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Clogher Valley, County Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=486. Accessed on 2026-11-06. |
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