ESCR - Glenelly/Owenkillew Valleys (Gortin)

Earth Science Conservation Review

Summary Full report
Glenelly/Owenkillew Valleys (Gortin)	Tyrone

Site number:	475
ESCC:	IS
Locality Type:	Inland exposure	Status:	TBC
Grid Reference:	H493858 Approximate
Google maps:	54.71825,-7.23496
County:	Tyrone	Council area:	Fermanagh & Omagh District Council
Period:	Quaternary

Site Description
Highlights:
The Glenelly/Owenkillew Valleys complex is located along the valley floor and margins of 400m deep east/west trending Owenkillew and Glenelly river valleys at the southern margin of the Sperrin mountains. The most scientifically and aesthetically important assemblage of deglacial landforms contained in the area is the glaciolacustrine deltaic complex at Gortin. The multiple levels of the delta tops show the deposition of glaciofluvially transported sediment into a proglacial ponded waterbody of varying elevation. Cross-valley morainic belts and ridges in the Owenkillew and Owenreagh valleys constrain the position of ice-margins during ponding of proglacial lakes at 256, 210 and 180m O.D. Possible overflow channels, the elevations of the lower two lake levels, occur in the mountain ridge forming the northern margin of the Glenelly valley. Deposition of the delta at the ice-margin is demonstrated by ice-contact slopes, kettles, the junction of subglacial feeder esker ridges at the rear of the delta tiers and glaciotectonic sediment deformation. The deeply incised Altaravan Glen channel immediately south of the esker ridges shows that sediment was transported from the south by subglacial meltwater at high velocity during the period that ice covered the Mullaghcarn massif. The deposits at Gortin and along the Glenelly valley have experienced minimal sand and gravel excavation. The complex at Gortin forms a glaciolacustrine sand and gravel reserve of probably unparalleled dimensions in the province. Glaciofluvial terraces, subaqueous fans and moraine ridges form a concentrated, overlapping belt of stratified deposits along the upper Glenelly river valley also adding significantly to its scenic value.
Description:
The deposits are divided into two genetically linked groupings confined spatially by the Glenelly and Glenlark/Owenkillew/Owenreagh valleys. AREA 1: GLENELLY VALLEY The Glenelly Valley encompasses an area from 4km southwest of Plumbridge (248300 391400) to Glengomna (271500 394200) west of the Moyola valley (see Key Site 474 - Foyle Valley Complex). The valley slopes from east to west (250m to 80m O.D.), and lies between the highland ridges of the Ballix Hill/Sawel Mountain/Crockbrack ridge (up to 680m O.D.) to the north and the Slievemore (250500 389700)/Slieveavaddy (269300 392300) ridge to the south (up to 562m O.D.). Both of which are composed almost entirely of the Glenelly upper and lower schists. The former ridge contains two rock-cut cols of lower elevation, the Butterlope Glen which intakes at 210m O.D. and the Letterbrat channel 1km further west, which intakes at 180m O.D. Both drain north into the Inver channel (248900 396400), part of the Burndennet drainage system. The Slievemore/Slieveavaddy ridge is punctuated in two locations along its length, at the steep-sided, rock-cut Barnes Gap (250m O.D.), and at the intake to the south/north sloping Sawelabeg channel (at 319m O.D.) at the head of the Glenlark valley . Up to an elevation of 270m O.D., from Lough Ouske (270600 393400) to Drumnaspar (253800 390400), is composed of glaciolacustrine and subaqueous spreads, cross-valley morainic ridges and glaciofluvial terrace fragments, all of which are poorly exposed. Perched valley-parallel moraine ridges form the highest deposits, occurring at up to 330m O.D. along the valley sides. The topographically distinct, valley-side moraine ridges occur as three main belts, at Lough Ouske, Sawelabeg (264700 393700), and Barnes Gap (255100 389800). 1. Lough Ouske A belt of up to 8m high and 0.75km long steep-sided morainic ridges, which decrease in elevation northwestwards from 330m O.D. to 260m O.D., occur on the northwest slopes of Carricknashinnagh mountain, impounding Lough Ouske (270600 393400) to the southeast. The ridges are dissected from the southeast, the direction of the Moyola valley. 2. Sawelabeg A northeast/southwest trending beaded, steep-sided, sharp-crested 2km long by 10-15m high ridge occurs at 310m O.D. along the steep northwest slopes of Mullaghbane (265800 392900). The ridge is dissected by several south/north sloping meltwater channels. 3. Barnes Gap Three southwest/northeast oriented sub-parallel ridges occur on the northwestern slopes of Mullaghbane (254700 389500) at 230-250m O.D., at the northwest margin of Barnes Gap. The ridges are up to 5m high and up to 0.2km long. Gently sloping, fan-shaped wedges of stratified sediment up to 50m thick occur along the Glenelly valley sides at the base of steep bedrock slopes forming the Carnanelly and Dart mountains. The spreads occur at two locations, immediately west of Goles forest (267500 394400) and at Cranagh (259000 392800). 4. Goles Forest A 0.5km2 south/north sloping, wedge-shaped deposit occurs on the southern side of the Glenelly valley between 220 and 285m O.D. from the western margin of Goles Forest as far as Carowoaghtragh (265600 3943000). The western (downvalley) margin is marked by a south/north sloping meltwater channel and small-scale cross valley ridge at up to 250m O.D. The toe of the fan is formed by a steep river-bank scarp. One poor exposure in the scarp, 15m from the valley floor, is composed of interbedded parallel bedded, well-sorted massive sand and poorly-sorted, matrix-poor cobble gravel. Sand units are up to 4m thick and individual cobble gravel beds up to 2.5m in thickness. A-axis imbrication of cobbles is to 30§N. The fan is bissected by the Altaloran Glen at (266700 394400). Two smaller scale spreads occur between 210 and 240m O.D. on either side of the channel. The deposits slope gently from south to north (towards the valley centre). No exposure occurs in the deposit. 5. Cranagh A 0.2km2 , gently sloping fan-shaped deposit occurs at the mouth of the Oughtboy Burn on the north side of the Glenelly valley between 150 to 190m O.D. The toe of the deposit forms the scarp of the present river channel. No exposure occurs in the deposit. 6. Dergbrough ridge A 0.6km long sharp-crested northwest/southeast oriented cross-valley ridge, 15m high which is dissected on its northwest margin by the Glenelly river occurs 2km southwest of Plumbridge town on the valley floor immediately west of the Cruckaclady mountain. The low relief ridge diverts the flow of the Glenelly river northwards by 0.3km. AREA 2 : GLENLARK/OWENKILLEW/OWENREAGH VALLEY 1. Glenlark moraines The Glenlark valley is up to 200m deep and drains southwestwards towards the Owenkillew valley. A train of small-scale cross-valley ridges and spreads occur along the valley axis between 200m O.D. in the southwest and 310m O.D. at Lough Lark (264600 392600)and at up to 290m O.D. along the valley sides. At Lough Lark, a hummocky belt ponds the lake on its southwest margin. A meltwater channel drains northwards into the Glenelly valley from the northern edge of the lake, dissecting the valley-side morainic ridge at Sawelabeg. 2. Fallagh Lower moraine A dissected sharp-crested, steep-sloped ridge spans the valley of the Owenreagh river at Fallagh Lower (254700 384800). The ridge top occurs up to 180m O.D. No exposures occur in the deposit. 3. Crosh moraine A 4km long by 200-300m wide arcuate belt (concave eastwards) of hummocky topography and mounds occurring up to 80m O.D. spans the lower reaches of the Owenkillew valley from Crosh House (241200 386800) to Milltown (243200 386800) to Moyle Glebe (242000 385500). 4. Gortin deltaic complex A well-defined landform complex composed of stratified drift occurs on the northern slopes of the Mullaghcarn and Curraghchosaly massif immediately south of Gortin town. From south to north, the complex begins with the rock-cut channel forming Altavaran Glen. Linear groups of hummocks and ridges segments which slope generally northwards for 0.5km across a relatively flat plain towards distinct, steep (>40§) slopes forming the southern margins of steep-sloped, flat-topped ridges and erosional remnants. Seven water-filled enclosed depressions occur on the flat-top of the 256m O.D. level and amid the linear ridge segments to the south. The flat-tops of the ridges occur at three topographic levels (256, 210 and 180m O.D.), with the 256m O.D. level the most extensive (0.3km2). The deposits are dissected by deep south/north sloping channels and have steep (35§), planar northern faces. The flat-topped segments located at 180m O.D. occur at the base of the steep-sided channel (Gortin Glen) which dissects the other two levels. Gortin delta exposure One exposure occurs in an erosional remnant of the ridge with a flat top located at 210m O.D. The mound is located at the southwestern margin of the complex (249000 384700). It is bordered to the east and west by steep meltwater channel sides, and to the south by a steep ice-contact slope and southeast/northwest oriented 2-3m high linear ridge segments. The section is 8m deep by 20 wide and consists of large-scale planar cross-bedded, well-sorted massive sand and pebble to cobble sand. Contacts between beds are generally transitional although occasional pebbly sand beds contain pebble lags. The beds dip at high angles (40§) to 040§N and show evidence of deformation by subparallel sets of shear planes which strike east/west and rise at 30§ to the northeast. The uppermost beds in the sequence are flat-lying, unsheared and occur above a sharp, uneven contact which cross-cuts the underlying sheared sediments. They consist of 0.7m of poorly-sorted cobble gravel with a well defined a-axis imbrication to 020§N.
Interpretation:
The lack of exposures prevents detailed palaeoenvironmental reconstruction of the full range of deglacial palaeoenvironments in this area. The general interpretation of the deglacial pattern in the area is inferred mainly from the relative location of landform assemblages e.g. morainic belts and meltwater channels, and comparison with known patterns of ice wastage in adjacent areas to the north and west. 1. Glenelly Valley The linear ridge morphology and valley parallel orientation of the deposits at Lough Ouske and the similar landforms on the opposite (northern) side of the valley, suggest that the landform assemblages represents the decay of an ice tongue into the valley bottom at the beginning of its westward retreat along the Glenelly valley. The west/east sloping flat-bottomed Glengomna channel and fluvioglacial terraces at Goles Forest further indicate meltwater discharge eastwards from a westward retreating valley glacier ice margin. The massive sands (remobilised) and poorly sorted cobble gravels (debris-flow) exposed at Goles Forest are interpreted as facies indicative of subaqueous sediment gravity flows down the valley-side slope. The formation of a fan-delta necessitates the presence of a ponded waterbody during deposition. The fan at Goles forest, formed from sediment input from an ice-margin retreating southwards across the Mullaghbane/Slieveavaddy ridge, was deposited into a localised waterbody in the upper reaches of the Glenelly valley at 285m O.D., impounded by decaying ice in the lower Glenelly to the west and the Moyola valley to the east. The lower elevation fan located at the western margins of the deposit are not exposed but are inferred to represent the reworking of sediments deposited during the formation of the Goles forest fan. A possible location of the ice-margin during its westward retreat along the lower Glenelly during deposition of the fan was at the Sawelabeg morainic complex, 1.5km to the west (downvalley). This moraine is perched on the valley side at 300m O.D. and indicates a period of either increased sediment flux to the margins of the valley glacier or a reduction the ice lobe retreat rates westwards along the valley. Similar lateral moraines occur at Lough Ouske and Barnes Gap. From these landforms it is inferred that ice retreat in the Glenelly valley was characterised by periodic variation in retreat rates along the valley and ice lobe downwasting into the valley bottom. Drainage of the lake eastwards into the Moyola valley along the Glengomna channel resulted in a lowering of the lake surface to 250m O.D., the elevation of the Glengomna channel intake. As ice retreat continued along the valley, continued influx of material into the proglacial waterbody ponded at 250m O.D., impounded by the reverse gradient of the valley and the retreating ice-margin, resulted in a complex pattern of morainic and glaciolacustine deposition along the valley floor and sides. Further morphological evidence, such as the incision of the ice marginal morainic ridges at Sawelabeg by south/north sloping meltwater channels and the formation of the fans at Goles Forrest and Cranagh, suggest that as an ice lobe retreated along the Glenelly valley, north and southward retreating ice-margins were also located on the valley sides. The slope of the fan shaped deposit at Cranagh suggests deposition occurred from meltwaters flowing north/south off the slopes of the Mullaghclogha/Sawel mountain ridge when a waterbody was impounded in the Glenelly valley at 210m O.D. This level may have been controlled by the Butterlope Glen meltwater channel located 9km to the west. The opening of this intake and thus control of lake level, resulted from westward retreat of ice margins along the valley. Further westward retreat opened a route for meltwater northwards along the Letterbrat channel, which intakes at 180m O.D. The cross-valley ridge at Dergbrough (247000 389800) was probably deposited in the lake at this time from sediment being transported eastwards from an ice margin located immediately west of Cruckaclady mountain. From the relatively thick accumulations of flat-topped glaciolacustrine sediments at Gortin to the south, located at these levels, the lake levels of 210m and 180m O.D. are inferred to have existed in this area for a considerable period of time. This leads to the conclusion that ice-marginal position was also relatively stable, with retreat rates significantly reduced. They also probably occupied the lower reaches of the Glenelly valley, resulting in the deposition of a subaqueous fan at Cranagh. 2. Owenkillew valley The Gortin deposits record a range of depositional environments at and near the margins of an ice lobe in contact with a ponded waterbody. These include ice-contact features, flat-topped deltaic ridges, oversteepened slopes at the rear of the flat-topped ridges, topographic inversion due to the melt-out of large buried ice blocks, glaciotectonic shearing of sediments and linear esker ridges feeding into the southward facing ice-contact slopes. The flat surface to the south of the ice-contact slopes probably represents the area occupied by the ice lobe during deposition of the high level deltas and therefore lacks thick sediment. A ponded waterbody at the ice-margin controlled the elevation of the upper depositional surface of the ice-contact ridges, resulting in flat surfaces at 256 and 210m O.D. A lower level at 180m O.D. was deposited as a subaqueous fan from erosion and reworking of the upper levels along a proglacial meltwater channel when water levels fell. The morphological pattern of other deposits and features in the area suggests that the highest lake level was localised in the Owenkillew valley. The lower two levels represent the elevation of lakes which probably extended northwards into and along the Glenelly valley as a result of westward retreat of an ice lobe towards the Mourne/Strule valley. SUMMARY The events in the circum-Gortin/Glenelly area are summarised in chronological order, starting with the ice advance pattern across the Sperrin mountain range. Stage 1: Ice pressure patterns Ice pressure form the southwest (Omagh basin) resulted in southwest/northeast migration of ice north of the Glenelly valley, with little control by topography upon the general direction of ice movement (Colhoun, 1970). Barnes Gap, and possibly Altavaran Glen, from their deep incision into bedrock are inferred to have formed along south/north flowing, high energy subglacial meltwater conduits. Stage 2: Ice downwastage initial patterns Ice mass down and backwastage southwards across the Sperrins resulted in the splitting of the ice mass across the Slievemore/Slieveavady ridge and strong topographic control on ice movement directions and the presence of ice masses in the topographic depressions. The thickest ice in the region was located to the west in the Strule valley and in the Omagh Basin to the south. The ice mass to the west filled the Glenelly valley above the level of the intake of the Butterlope Glen and Letterbrat channel (intakes to the Inver Channel). Ice tongues, now lower than the depth of the upper valleys shown by the Lough Ouske, Sawelabeg and Barnes Gap moraines, retreated generally westwards along the Glenelly, Glenlark, lower Owenkillew (to the mouth of the Glenlark valley) valleys, and southwards into the Mullaghcarn uplands and along the Owenreagh valley. Ice within the Owenreagh and Owenkillew had split into generally westward and southwestward retreating tongues, strongly controlled by the massifs of Mullydoo/Crocknamoghil and Mullaghcarn. Stage 3: Lake formation A lake ponded to 256m O.D. in the Owenkillew valley around Gortin, constrained by three ice margins in the lower Owenkillew (Crosh moraine), the Owenreagh valley (Fallagh Lower moraine) and upper Owenkillew. The presence of ponded water is shown by the glaciolacustrine sand and gravel complex at Gortin and 'heavy silts' in the valley bottom at Glenmacoffer Bridge (252800 385700) and Freaghdoo (252100 385900) (Colhoun, 1968; p281). The opening of a pathway to the Butterlope Glen channel by downwasting of the valley glacier occupying the Glenelly valley, resulted in a drop of the lake level to 210m O.D. Further downwasting and westward backwasting opened a pathway to the Letterbrat channel and a decrease in elevation of the lake level to 180m O.D. OVERVIEW Several large lakes formed in the area of the Owenkillew valley during the deglacial period. Deposition into the lakes at the margins of the Mullaghcarn highlands resulted in the deposition of large glaciolacustrine deltas which record successive falls in lake levels (256, 210 and 180m O.D.). Falls in lake level were due ice-mass downwasting and retreat during deglaciation and the opening of pathways to lower elevation intakes, which carried meltwater northwards.
Notes:
IMPORTANCE Landforms in the upper Glenelly valley are relatively intact examples of deglacial landforms which have probably remained so due to their high altitude location with little reworking by fluvial erosion and their formation near the end of the last glacial cycle. Generally, the perched morainic ridges and outwash spreads document the withdrawal of ice margins south and westwards from the spine of the high Sperrins to the north. The Gortin deltaic complex forms one of the largest scale examples of a proglacial delta landform assemblage in Northern Ireland. The complex contains all of the landform elements in classic models of the assemblage, including a flat-topped delta, steep ice-contact slopes, flat subglacial surfaces, kettle holes, steep delta foreslopes, post-depositional dissection channels and subglacial Nye-type feeder channels. Following the classic model of Gilbert-type delta formation, the flat-top of the delta tiers implies that these were the levels of a waterbody impounded in the Owenkillew valley during the time of ice presence in the highlands to the south. One exposure located in a remnant of the second tier shows the composition of the landform, the sediment geometry, the palaeoflow of the efflux which formed the delta and the direction of ice pressure from sediment shears. The complex is spatially confined and through its demonstration of landform association, of excellent educational value. The palaeoflow of the meltwater, which transported the sediments and later dissected the deltas, shows pressure from an ice-mass located to the south of the deposit, in the Omagh basin. This provides further evidence for the assertion that the Omagh basin was one of the last decay centres for ice at the end of the last glaciation (see Key Site 473 - Dunnamanagh, and Key Site 474 - Foyle Valley Complex). 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:	Glacio-fluvial sediment
Geomorph:	esker, flat-topped delta, kettle-hole, moraine, Nye-type channel, subaqueous fan
Paleoenv:	ice contact

Access
Management:
The unit is located at the southern margins of the main Sperrin mountains range. The area is a rural highland one with cultivated land along the present misfit river valleys forming a skirting to the heather covered uplands of the range. Several locations within the area are both scientifically and aesthetically important. The high scientific importance, scale, accessibility and prominence along the route of the Ulster Way of the Gortin complex are significant attributes in an assessment of the region. The large deltaic ridge is a topographically unique feature in Northern Ireland, in respect of its well defined ice-contact slopes, flat-top and foreslopes. Vistas of the Sperrin mountains to the north from the tourism facilities located on the upper tier of the delta are magnificent. Little disturbance of the delta for commercial aggregate production has occurred, except at one location (which has a strong case for management for educational and tourism purposes). This results in little detraction from the scenic topographic diversity added to the local environment by the ice-contact landforms. The landform probably contains a significant reserve of stratified sand and gravel and is possibly vulnerable to commercial exploitation in the future. Prominent, perched moraine ridges occur at Sawelabeg (at 300m O.D.) and Lough Ouske (at 250-340m O.D.) in the upper reaches of the Glenelly valley. Large, dissected spreads of sand and gravel with gently sloping, lobate top surfaces and truncated valley-floor margins add further topographic diversity to the valley.

Access

Management:

The unit is located at the southern margins of the main Sperrin mountains range. The area is a rural highland one with cultivated land along the present misfit river valleys forming a skirting to the heather covered uplands of the range. Several locations within the area are both scientifically and aesthetically important. The high scientific importance, scale, accessibility and prominence along the route of the Ulster Way of the Gortin complex are significant attributes in an assessment of the region. The large deltaic ridge is a topographically unique feature in Northern Ireland, in respect of its well defined ice-contact slopes, flat-top and foreslopes. Vistas of the Sperrin mountains to the north from the tourism facilities located on the upper tier of the delta are magnificent. Little disturbance of the delta for commercial aggregate production has occurred, except at one location (which has a strong case for management for educational and tourism purposes). This results in little detraction from the scenic topographic diversity added to the local environment by the ice-contact landforms. The landform probably contains a significant reserve of stratified sand and gravel and is possibly vulnerable to commercial exploitation in the future. Prominent, perched moraine ridges occur at Sawelabeg (at 300m O.D.) and Lough Ouske (at 250-340m O.D.) in the upper reaches of the Glenelly valley. Large, dissected spreads of sand and gravel with gently sloping, lobate top surfaces and truncated valley-floor margins add further topographic diversity to the valley.

References (2)
Colhoun, E.A., 1968. The glacial geomorphlogy of the Sperrin Mountains and adjacent areas in Co. Tyrone, Co. Londonderry and Co. Donegal, Northern Ireland (Thesis) Unpublished PhD Thesis, Queen's University, Belfast.
Colhoun, E.A., 1970. On the nature of the glaciations and final glaciation of the Sperrin Mountains and adjacent areas in the north of Ireland Irish Geography, vol. 6 pt. 2, pp.162-185.

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

Enlander, I., Dempster, M. & Doughty, P., 2026. Glenelly/Owenkillew Valleys (Gortin), County Tyrone, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=475. Accessed on 2026-11-06.