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Earth Science Conservation Review
| Murrins Complex - Overview | Tyrone |
| Site number: | 479 | ||
| ESCC: | IS | ||
| Locality Type: | Inland exposure | ||
| Grid Reference: | H612725,H510810,H595828 Corners of site, Not Entered | ||
| Google maps: | 54.59749,-7.05311 | ||
| County: | Tyrone | Council area: | Fermanagh & Omagh District Council |
| Period: | Quaternary | ||
| Site Description | |
|---|---|
| Highlights: | |
The Murrins complex records a sequence of sedimentary events associated with southwestern ice retreat from the southern Sperrin Mountains into the central part of the Omagh Basin. Sediment morphologies and inferred depositional settings include subglacial eskers, ice marginal moraines and proglacial outwash and deltas. These sediment types demonstrate fundamentally that the ice margin was highly dynamic and characterised by a high outflow of sediment and meltwater. Eskers are generally well preserved but have little exposure. Moraines, outwash and deltas have highly variable sedimentary characteristics but have been exploited by sand and gravel aggregate extraction. These features are therefore valuable as an economic as well as a landscape resource. | |
| Description: | |
North to south, the solid geology of this area can be split into three units: the Mullaghcarn unit, the Tyrone Igneous Complex, and the Sixmilecross (H571680) unit. The uplands around Mullaghcarn in the northwest of the study area one composed of Upper Dalradian quartz-mica-schists (Wilson, 1972). This unit is separated from the Tyrone Igneous Complex (TIC) by the northeast-southwest trending Omagh Fault, a major Caledonian lineament (Ryan et al., 1995). The TIC and associated metamorphics (Hartley, 1933) covers the central and northeastern study area. Its southern limit extends from Termon (H625710) to Drumduff (H546703). The TIC is composed of an intrusive suite of Ordovician and Dalradian age metasediments, basic plutonic rocks and granites (Wilson, 1972) and include tuffs, quartz porphyry, diorite, andesite and gneiss (Hartley, 1933). The area around Sixmilecross is composed of Devonian ORS and associated conglomerates (Wilson, 1972). The Sixmilecross/Pomeroy valley south of Carrickmore is dissected by the Tempo-Sixmilecross Fault which trends northeast-southwest. Like the Omagh Fault, this has exerted a strong influence on topography and glacial history. Glacial sediments in the Murrins complex are found generally below 240m elevation and can be split into 5 morphological regions on the basis of sediment type, morphological expression, deglacial history and association with adjacent sediments. These areas are (1) Glensawisk Burn delta, (2) Cashel outwash, (3) Lough Fingrean frontal moraines, (4) Laght Hill retreat moraines, and (5) Coolesker/Beragh eskers. For site specific information see the following sites: Key Site 508 - Murrins Complex - Glensawisk Burn Delta Key Site 509 - Murrins Complex - Cashel Outwash Key Site 510 - Murrins Complex - Lough Fingrean Frontal Moraines Key Site 511 - Murrins Complex - Laght Hill Retreat Moraines Key Site 512 - Murrins Complex - Coolesker/Beragh Eskers | |
| Importance: | |
The Murrins moraine-outwash complex is of unique importance on a Northern Ireland scale by virtue of its morphological and sedimentary characteristics. This is because: A) The landform assemblage (esker - moraine - outwash) is well developed and intact. B) Landform size, dimensions and orientations give rise to visually prominent landscapes. This is accentuated by the juxtaposition of different landscape elements, such as meltwater channels, kettleholes and moraines. C) The juxtaposition of different substrate types, such as bedrock, diamict, and sand and gravel. D) Rock rafts appear to be unique to this area, and have not been described from Northern Ireland before. They infer a glacial readvance from the north, which would require a reassessment of the deglacial dynamics of this area. E) Kettleholes and peatlands support a variety of valuable ecosystems. | |
| Interpretation: | |
The Murrins, in contrast with other areas in Northern Ireland composed of glacial sediment, has been extensively studied at a range of levels and scales (Charlesworth, 1924; Colhoun, 1970; Dardis, 1980, 1982, 1986; McCabe et al., 1992). This provides the framework for future investigation and interpretation. Dominant morphological characteristics of the Murrins area included delta and outwash surfaces, moraines and eskers. Sedimentary sections reveal that a variety of subaqueous and ice-marginal environments were responsible for these morphological complexes. Further, they indicate that an Omagh Basin ice margin retreated southwest and that other ice margins were periodically important. In succession these periods are: (1) Glensawisk Burn delta deposition. This requires a blocking ice margin somewhere across the Owenreagh valley. (2) Beragh Hill and Coolesker ridge deposition, and stagnation topography development. This may be related to ice from the Fintona Hills ice emerging eastwards through the Pomeroy valley. (3) Rock raft emplacement. Rock raft sites (discussed later) record an ice margin pushing south from the Mullaghcarn area. Southwestern retreat of the Omagh Basin ice mass was accompanied by a high sediment flux to the ice margin. This gave rise to the dominantly constructional morainic landscape. Dardis's (1982, 1986) extensive study of the Murrins area is now used as a paradigm against which the features and sediments described here are interpreted. Dardis (1982, 1986) believed that subaquatic sedimentation in the Murrins complex took place in a series of impounded water bodies developed between the bedrock uplands of the southern Sperrin Mountains to the north, the Fintona Hills to the south and the margins of the retreating Omagh Basin and Lough Neagh Basin ice masses to the west and east respectively. Characteristics of Dardis's (1982, 1986) lakes are as follows: (1) The location of and water constituting these glacial lakes were generated by the pull-apart of the Omagh Basin and Lough Neagh Basin ice masses. (2) Lake levels were controlled by the uncovering of bedrock-floored spillways during ice retreat. Dardis cites five main lake levels (between 245-187m a.s.l.) in the Murrins area. (3) Lakes were fed by, or associated with, small feeder eskers. Further, Dardis (in McCabe et al., 1992, p.109) interpreted the association of deltaic, lacustrine and fluvial components as a Salisbury-type overwash plain (cf. Church and Gilbert, 1975, p.86). Dardis's (1980, 1982, 1986) interpretation is largely accepted here. A Salisbury-type overwash plain (Church and Gilbert, 1975) can account for a variety of deposits. However, the Murrins landform assemblage is retrogressive inasmuch as the oldest sediments are in the north and the youngest in the south, relating to southward ice withdrawal. Therefore sediment progradation and foreset development is only localised and related to local ice margin stillstands during general retreat. However, there are six main problems with Dardis's (1982, 1986) glacial lakes model: (1) There is no evidence for large proglacial lakes. No regional drapes, rhythmites, shoreline notches or subdued submerged topography are seen. (2) It is unlikely a large lake could be buttressed stably by east and west ice margins, particularly if those margins are extensive and active. (3) Lack of evidence for deep drainage channels favours a number of local lakes rather than regional ones. This is reinforced by sedimentary evidence (i.e. at Cashel Bridge). (4) Active ice margins of Omagh Basin ice, Sperrin ice to the north (blocking the Owenreagh valley and possibly involved in rock raft emplacement) and Pomeroy valley ice (impounding the Beragh Hill delta) are unaccounted for in Dardis's model. (5) Contrary to what Dardis (1982) described from the Murrinmaguiggan pit, there is no evidence for extensive subaqueous resedimentation, massflow diamicts or chutes. (6) Feeder eskers probably contribute only part of the sediment flux to the margin, and may not be related to frontal moraine build-up as at Lough Carn (discussed later). A general deglaciation model for the Murrins area is now developed. The Cashel Bridge exposures show a series of large-scale foreset beds developed at c. 160m a.s.l. This is approximately the same height as the lowermost surface of the Glensawisk Burn delta at Fallagh Upper. Therefore the Owenreagh valley served as a bedrock-floored deep water depocentre for deltaic foreset sediments. If a topset facies ever developed it has been destroyed by later shallow water reworking as ice retreated south. The Omagh Basin ice margin must have been located around Cashel Rock during foreset deposition, with the lower and middle Owenreagh valley blocked to the west by Foyle ice protruding through the valleys (cf. Colhoun, 1970). Water shallowing during ice retreat, as revealed by a lack of deep foresets and dominance of outwash sediments in the Cashel area, is due to the broadening of the lowland area within which any lake would have been impounded. It is likely therefore that the bedrock surface is also flat around Cashel and that no deep bedrock basin is provided. Lack of moraines in this area can be attributed to (a) rapid ice retreat rates where the ice margin is not pinned against bedrock protuberances, and (b) likely sediment reworking of any landform produced during retreat. Reworking probably took place all along the ice front, and was not restricted to where water and sediment was pushed from the system (i.e. from feeder eskers according to Dardis [1980, 1982]). Rapid retreat rates in this area are also evidenced by ice block detachment and kettlehole formation, related to the location of the active ice front within the ice mass (Mulholland, 1982). These kettleholes (i.e. Lough Carn) are generally small and front the main moraine ridges. Moraine formation, associated with ice marginal stillstands, result from topographic pinning against the ring of bedrock highs surrounding the Cashel outwash area, such as against Mulnafye and Loughmacrory Hill (Key Site 509). This interpretation implies that these hills were probably deglaciated (cf. Charlesworth, 1924), as evidenced by their constraint upon the location of retreat moraines (Key Site 511), and that the ice margin was characterised by a number of lobes. One problem with individual ice lobes interfingered between bedrock highs (Charlesworth, 1924) is that the size and continuity of the frontal moraines (Key Site 510) require a high sediment flux. This flux must therefore have been concentrated between these highs, either through feeder eskers (cf. Dardis, 1982) or as more dynamic and short-lived sediment transport events in the subglacial zone right across the ice margin. (1) The Mountfield area between Mulderg and Mulnafye shows feeder eskers but lacks a frontal moraine and kettled outwash topography. A concentrated subglacial outflow may not have given rise to either a frontal moraine or ice stagnation, i.e. that basal ice was clean and remained active. Clean and active ice retreat may also preserve esker morphology and not have led to proglacial reworking or addition of melt-out till facies (Paul and Eyles, 1990). This contrasts with (2) the Lough Carn/Lough Fingrean area where bedrock valleys to the south may have been scoured of sediment or overdeepened by a high subglacial sediment flux across a broad margin. This probably promoted the formation of frontal moraines and kettleholes. Therefore basal ice may have been 'dirty' and characterised by shearing-up between active and stagnant ice components (Mulholland, 1982), promoting sediment reworking (cf. Lough Carn pit). Retreat of dirty ice into separate lobes between high ground areas (Charlesworth, 1924) may have had two effects: (1) provided the sediment for prominent retreat moraines, as around Laght Hill (Key Site 511), and (2) caused the disaggregation of ice blocks as the lobes separated. This model of landform development can adequately account for morphological and sedimentary diversity in this area. Other features of importance in the Murrins complex include rock rafts, meltwater channels and kettleholes. Rock rafts are unique to this area, having not been observed anywhere else in the Omagh Basin region. They are found in 6 locations: Mountfield quarry (H544789), Mountfield, Murrinmaguiggan, Maine (H517753), Mullanmore (H5987521) and Glen Upper (H551764). Rock rafts vary between 0.2-25m long and 0.05-2m high. They are usually separated or bordered by a mylonitised zone of macerated bedrock and occur as stacked successions up to 5m high, often interbedded with crushed bedrock breccias or coarse angular diamicts. General characteristics of rock rafts in this area include a topographically high location, northern raft provenance and inferred emplacement direction, association with intraformational diamict and a stratigraphically high position in the profile. Glaciotectonic detachment, transport and emplacement of glacial rafts or megablocks has been attributed to processes of beheading, plucking and scooping associated with the development of high subglacial fluid pressures (Aber, 1985). Once enclosed within basal ice, changes in ice compression/extension and glaciotectonic comminution determine the geometry, deposition location and likelihood of raft survival as they are carried from the subglacial zone to the ice margin (Aber, 1985; Croot, 1987). Rafts deposited subglacially, i.e. by lodgement, are generally flat-lying and include folds, brecciation, rock smears, detachment and directional indicators characteristic of post-depositional overriding. Lodgement till beds may also occur within or above the rock rafted zones. Most of these characteristics are found in the Maine pit which show that ice flowed north to south depositing then deforming a rafted sequence above which a basal diamict was deposited. Rafts that are deposited at the ice margin may be glaciotectonically thrust into an imbricate and faulted push moraine morphology by compressional deformation and glaciotectonic shortening. Rock rafts at this location therefore dip towards the direction of the ice margin. This is shown in the Murrinmaguiggan pit where a large rock raft dips (few degrees) to the north (000ยง) and is contained within a carapace of clay matrix-dominant diamict. This diamict may have aided marginal thrusting by providing a shear plane for basal decollement. It may also have helped the rock raft to remain intact. The Murrinmaguiggan raft indicates that the ice margin responsible for raft emplacement was (1) derived from the north, (2) did not advance further than the raft site, and (3) was one of the last ice oscillations in the region, and one not identified by Charlesworth (1924), Colhoun (1972) or Dardis (1980, 1982, 1986), mainly through the lack of geomorphic expression. Meltwater channels are also a marked feature of the region. Channels relate in age and direction to glacial retreat stages. Channels dominate where water flow was concentrated either by adjacent bedrock uplands, or by the position of the ice margin. Meltwater channelling incises positive topographic elements such as the Glensawisk Burn delta, frontal moraine ridges at Murrinmaguiggan and retreat moraine ridges between Mountfield and Mulnafye, and between Mulnafye and Laght Hill. Channels are up to 60m deep, and generally broaden and shallow to the north in all cases. Landform incision is important in providing topographic contrast, which adds to landscape diversity. Kettleholes associated with the detachment of ice blocks from a deglaciating ice margin during retreat, are also important landscape features. Decreasing kettlehole size to the north, and association with the outwash of the Cashel area, indicates contemporaneous kettle formation with sediment deposition (Mulholland, 1982). This association must reflect the last glacial event in this area since glacial advance destroys buried subjacent ice (Bluemle and Clayton, 1984). Kettleholes in this area are important geomorphic, topographic and landscape features. They add ecosystem diversity and contribute to landscape quality. The high level lake of Lough-a-Tirrive occupies a bedrock hollow or scour, therefore is not a kettlehole although all other loughs are. Loughs Fingrean and Macrory are the largest kettle lakes in the area and cover about 0.4km2 each. An interconnected area of small kettleholes at (H575795) ['Brackagh South' of Dardis in McCabe et al. (1992, p.112)] was destroyed at or before 1985 when this pit was last active. On the basis of the geomorphic and sedimentary evidence outlined above, the following sequence of events is suggested for the Murrins area: 1) Late Devensian ice thinning and breakup into Omagh Basin, Lough Neagh Basin and Sperrin ice masses, possibly over Evishanoran to the east of this area. 2) Deposition of the Glensawisk Delta (235-160m a.s.l.) from an ice source near Lough-a-Tirrive into a water body impounded by Foyle ice around Fallagh Upper. 3) Probable retreat of Foyle ice westward along the Owenreagh valley, Omagh Basin ice margin around Cashel Rock, and the deposition of the Cashel Bridge delta (160m a.s.l.). 4) Southward retreat of Omagh basin ice and the development of a Salisbury-type overwash plain (180m a.s.l.) around Cashel. 5) Frontal moraine formation from an ice margin between Mulderg and Loughmacrory Hill. Meltwater dissection followed. 6) Mullaghslin delta formation (160m a.s.l.) probably related to supporting ice margins to the northwest and southwest. The Seefin esker may have fed this system, and may have been ice-marginal in origin rather than subglacial. 7) Retreat moraines and stagnation topography around Lough Macrory. Possible deglaciation around Mountfield. 8) Beragh Hill (140m a.s.l.), Coolesker and adjacent stagnation topography formation, possibly associated with (feeder) eskers near Annagh and Gortaclare. This stage, and stage (6), indicates that the entire eastern Omagh Basin ice margin from Mullaghcarn to the Fintona Hills was in retreat. 9) Rock raft emplacement from the Mullaghcarn area. This supports the hypothesis that the Mountfield area was deglaciated and that ice could advance rapidly southwards. 10) Omagh Basin ice retreat to the southwest, uncovering of feeder eskers. 11) Postglacial landscape development: raised bogs, kettlehole systems, some lowland alluviation. | |
| 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: | Glaciolacustrine deposit, Ice retreat |
| Geomorph: | deltas, esker, kettle-hole, moraine, outwash, rock raft |
| Measurements | |||||
|---|---|---|---|---|---|
| Length: | Width: | Height: | |||
| Depth: | Area: | 100km2 | |||
| Access | |
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| Management: | |
This area is characterised by generally intact and visually dominant aligned ridges interspersed with flat outwash spreads. The major pit at Murrinmaguiggan seriously despoils the local landscape. The former Brackagh South pit (Dardis, in McCabe et al., 1992) is very extensive and of low landscape quality. It has not been worked for about 10 years but has not been re-seeded, landscaped or reclaimed in that time. This is a serious issue of bad landscape management that should be avoided in the future. Specific aesthetic landscape attributes include: 1) The visual intactness of the overall landform assemblage, particularly the Glensawisk Burn delta, which is enhanced by meltwater incision. 2) The frontal moraines around Lough Carn are very important features on a Province-wide level. They contrast strongly with the forefield of glacial outwash, and are prominent from the main Omagh-Cookstown road. 3) Esker ridges are generally intact and are well-developed. The Coolesker ridges and the Seefin ridge are very good examples. They contrast in morphology and orientation to the surrounding peatlands and diamict ridges. | |
| Uses | |
| Uses: | Some sand and gravel quarrying |
| References (18) | |||
|---|---|---|---|
| Aber, J.S., 1985. The character of glaciotectonism Geologie en Mijnbouw, vol. 64, pp.389-395. | |||
| Benn, D.I., 1992. The genesis and significance of 'hummocky moraine': evidence from the Isle of Skye, Scotland Quaternary Science Reviews, vol. 11, pp.781-799. | |||
| Bluemle, J.P. & Clayton, L., 1984. Large-scale glacial thrusting and related processes in North Dakota Boreas, vol. 13, pp.279-299. | |||
| Boothroyd, J.C. & Ashley, G.M., 1975. Processes, bar morphology and sedimentary structures on braided outwash fans, northeastern Gulf of Alaska Society of Economic Palaeontologists & Mineralogists Special Publication, vol. 23 In Glaciofluvial and Glaciolacustrine Sedimentation. | |||
| 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. | |||
| Church, M. & Gilbert, R., 1975. Proglacial fluvial and lacustrine sedimentation Society of Economic Palaeontologists & Mineralogists Special Publication, numb. 23 In Glaciofluvial and glaciolacustrine sedimentation, pp.22-100, Tulsa, Oklahoma. | |||
| 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. | |||
| Croot, D.G., 1987. Glacio-tectonic structures: a mesoscale model of thin-skinned thrust sheets? Journal of Structural Geology, vol. 9, pp.797-808. | |||
| Dardis, G.F., 1980. The Quaternary sediments of central Ulster IQUA Field Guide, numb. 3 In County Tyrone, Northern Ireland, pp.5-29, Dublin. | |||
| 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. | |||
| Dardis, G.F., 1986. Late Pleistocene lakes in south-central Ulster, Northern Ireland Irish Journal of Earth Sciences, vol. 7, pp.133-144. | |||
| Hartley, J.J., 1933. The geology of north-eastern Tyrone and the adjacent parts of County Londonderry Proceedings of the Royal Irish Academy, vol. 41 pt. B. | |||
| McCabe, A.M., Dardis, G.F. & Hanvey, P.M., 1992. Glacial sedimentology in northern and western Ireland: pre- and post-symposium field excursion guide book, pp.236PP Anglia Polytechnic, Cambridge. | |||
| Mulholland, J.W., 1982. Glacial stagnation zone retreat in New England: bedrock control Geology, vol. 10, pp.567-571. | |||
| Paul, M.A. & Eyles, N., 1990. Constraints on the preservation of diamict facies (melt-out tills) at the margins of stagnant glaciers Quaternary Science Reviews, vol. 9, pp.51-69. | |||
| Rust, B.R. & Romanelli, R., 1975. Late Quaternary subaqueous outwash deposits near Ottawa, Canada Society of Economic Palaeontologists & Mineralogists Special Publication, numb. 23 In Glaciofluvial and glaciolacustrine sedimentation, pp.177-192, Tulsa, OK. | |||
| Ryan, P.D., Soper, N.J., Snyder, D.B., England, R.W. & Hutton, D.H.W., 1995. The Antrim-Galway Line: a resolution of the Highland Border Fault enigma of the Caledonides of Britain and Ireland Geological Magazine, vol. 132, pp.171-184. | |||
| 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. Murrins Complex - Overview, County Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=479. Accessed on 2026-11-07. |
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