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Earth Science Conservation Review
| Tempo Valley Delta Complex - Overview | Tyrone |
| Site number: | 485 | ||
| ESCC: | IS | ||
| Locality Type: | Inland exposure, Pit | ||
| Grid Reference: | H398542,H302411 Corners of site, Not Entered | ||
| Google maps: | 54.43524,-7.38665 | ||
| County: | Tyrone | Council area: | Fermanagh & Omagh District Council |
| Period: | Quaternary | ||
| Site Description | |
|---|---|
| Highlights: | |
The Tempo Valley records a range of ice marginal and ice contact sediments including glaciolacustrine outwash spreads, deltas and moraines. These deposits are superimposed upon bedrock in the uplands, and subglacial diamict of rogen moraine morphology in the lowlands. These rogen bedforms relate to earlier northeast-southwest ice flow from ice centres north of the Fintona Hills. Ice retreated east and southeast from the main axis of the Fintona Hills. Three exposures in the Tempo Valley, at Dooneen (H390530), Tonyglaskan (H383528) and Pubble (H369461) show well-sorted and well-bedded sand and gravel facies, typical of subaqueous environments. The Dooneen exposure shows thick sequences of well-sorted medium and fine-grained sand deposited in an environment unaffected by rapid changes in sediment input. The Tonyglaskan exposure shows well-sorted sands overlain by high-density, chaotic boulder beds characteristic of an ice-proximal environment. The Pubble exposure records the deposition of rhythmically bedded sand, gravel and clay/silt beds in a small impounded lake. Deep meltwater channels through the axis of the Tempo Valley probably reflect meltwater and/or glacial lake drainage to the southwest. These exposures show that ice retreated generally south and east from the highlands, impounding proglacial lakes between the bedrock upland of Brougher Mountain-Topped Mountain and the ice margin to the south and southeast. Moraine ridges around Lough Eyes (H325434) record ice activity and the carriage of sediment to the ice margin, associated with a marginal stillstand-phase during general ice retreat into the lowlands. This stillstand is recorded by the position of the morainic complex at the extreme southwest of the Fintona Hills. The Lough Eyes moraines have diverted the course of the postglacial Tempo River. | |
| Description: | |
Physiography of the Tempo Valley is largely controlled by the Tempo-Sixmilecross Fault, which runs northeast-southwest along the axis of the valley. This fault, of Caledonian age (Wilson, 1972, Ryan et al., 1995), divides the area into two geological sectors. To the northwest of the fault, Devonian ORS composes the Fintona Hills sector. This sector is characterised by bedrock hills up to 316m in elevation (at Brougher Mountain [H349528]) with a surficial covering of glacial diamict and postglacial peat. A rib of ORS-age conglomerate also trends northeast-southwest through Brougher Mountain and Ballyreagh (H315495). The sector southwest of the fault is underlain by Carboniferous clastics, limestones and shales, mainly of Vis‚an age. This area, 80-100m in elevation, is characterised by a series of undulating diamict and sand and gravel ridges interspersed with peatlands. Northeast-southwest oriented Tertiary dolerite dykes occur in both sectors, but have had very little effect on the process and pattern of glaciation. The Tempo Valley complex can be split into four morphological areas on the basis of sediment type, landform orientation, relative-age relationship between adjacent landforms, and their inferred depositional setting. North to south these areas are: (1) the area between Glengesh and Tattinweer, which includes the Dooneen and Tonyglaskan exposures, (2) the area around Tempo, which includes the Pubble exposure, (3) the area around Lough Eyes, and (4) the upland areas of the Fintona Hills. The lowland diamict ridges, interpreted as rogen moraine, are discussed in the Clogher Valley site (Key Site 486), and are not considered here. For site specific information see the following sites: Key Site 513 - Tempo Valley Delta Complex - Glengesh and Tattinweer Key Site 514 - Tempo Valley Delta Complex - Tempo and Pubble Key Site 515 - Tempo Valley Delta Complex - Lough Eyes Key Site 516 - Tempo Valley Delta Complex - Upland areas of the Fintona Hills | |
| Importance: | |
The Tempo Valley complex is of high importance by virtue of its location, extent, and morphological and sedimentary characteristics. This is because: A) It acts as a link between the sand and gravel areas of the Fintona Hills and the Clogher Valley. There are shared morphological components, such as flat outwash/delta surfaces, meltwater channels and elongate ridges. B) Sedimentary characteristics, including concretions and mud curls, are unusual in glacial deposits and have not been described in detail from Northern Ireland before. Additionally, the sedimentary hosts are very well-sorted and bedded. C) The Lough Eyes moraine complex is unusual in its extent and ridge size. It is a unique feature in a Northern Ireland context. D) Marginal meltwater channels are uncommon in Northern Ireland, but are usually associated with the foothills of upland areas. Meltwater channels in the Tempo Valley are of large size. E) Deglaciation patterns confirm that (i) ice moved off the Fintona Hills, (ii) marginal sediment complexes were built up, (iii) the Upper Lough Erne Basin was largely deglaciated (McCabe, 1969), and (iv) ice also retreated westward into the Brougher Mountain/Ballyreagh area (Charlesworth, 1924), possibly into the Trillick valley. Further, two ice masses were involved in the formation and history of the Tempo Valley complex, and the deglaciation pattern is rather different to that of Charlesworth (1924). | |
| Interpretation: | |
Ice activity can be inferred from sediment texture, sorting, gross geometry and overall landform morphology. Well-sorted sand beds, continuous sedimentation, no glaciotectonic structures, delta dissection and lack of evidence for ice-marginal moraines in the north of the valley may indicate ice stagnation. Extensive (4km wide) moraine ridges around Lough Eyes (H325434) may indicate active ice-marginal retreat and a high sediment flux to the margin. Therefore there appear to be contrasts in the proxy record of ice activity in the Tempo Valley. The elevation of sand and gravel sediments in the Clogher Valley decreases down valley and reaches the approximate level of the watershed (along the east-west axis of the Fintona Hills) around Glengesh. In order to support an ice-dammed lake in the upper Tempo Valley it is uncertain whether (1) a northern ice margin at Glengesh is required, or (2) if one was not required whether it was likely to be present or not. Meltwater may have been discharged across this watershed from south to north. Evidence for this includes streamlined ridges, incised bedrock channels and a surficial litter of subrounded limestone boulders (<1.5m diameter) to the north of Glengesh. Similar evidence supporting glacial lake drainage has been reported in other glaciated areas (Kehew and Teller, 1994). These limestone boulders may have been carried onto the ORS bedrock across the Tempo Fault which dissects this area. Areas of sand and gravel deposition are constrained by the position of the ice margin at the time of sediment deposition. Upland areas over 150m are characterised by cols, generally aligned east-west, with associated marginal moraines, a lack of sand and gravel, and high level bedrock-floored lakes. These lakes may be related to both subglacial bedrock scouring and ice stranding or stagnation on exposed cols. Sand and gravel sediments form distinct morphological assemblages. (1) flat spreads against upland bedrock and (2) isolated hills and ridges separated by alluvial lowlands. Both these morphological assemblage types are dissected by meltwater erosion. Local lakes were impounded at c. 150m and c. 120m elevation in the northern part of the Tempo Valley, south of Brougher Mountain. These morainic and glaciofluvial sediments overlie diamict ridges which characterise the lowland area between Tempo and Brookeborough. These ridges are generally aligned northwest-southeast (i.e. perpendicular to the inferred direction of ice flow), and are interpreted as rogen moraine bedforms. Charlesworth (1924, p.271) names the Lough Eyes morainic complex the 'Glundees moraine'. He interprets ice decay as proceeding from east to west as ice retreated into the Esk and Glen valleys depositing arcuate end moraines. Meltwater from these receding ice margins flowed west to east and incised and reworked the morainic material. The 'Glundees moraine' is interpreted as having been emplaced from the west, associated with larger end moraines stretching northeast-southwest across the Clogher Valley. McCabe (1969) interprets the moraine system as having formed from lowland ice depositing material from the east. An exposure at Clundees Hill (sic) is dominated (81%) by ORS clasts with a northern provenance (McCabe, 1969). This interpretation is largely followed here. The following sequence of events in the Tempo Valley is suggested on the basis of the geomorphic and sedimentary evidence outlined above: 1) Generally northeast-southwest iceflow during the Late Devensian. This direction is inferred from Rogen moraine orientation in the Tempo/Clogher valley lowlands (see Key Site 486 - Clogher Valley). 2) Ice breakup over the Fintona Hills. It is likely that this breakup affected only the main hill ridge, not necessarily those considered here (Brougher Mountain, Ballyreagh, Topped Mountain etc). This is inferred from evidence of high level cols and westward retreat with marginal moraine formation. Also a Brougher Mountain ice mass must have been present to produce the chaotic boulder facies observed at Tonyglaskan (exposure A). 3) Glacial lake formation at c. 150m at Tonyglaskan with the Tonyglaskan delta built up. If meltwater or lake drainage occurred across the watershed through Glengesh, then water levels may have been even higher (between 170-180 m). The Tonyglaskan delta was emplaced by ice in the uplands to the northwest around Brougher Mountain (Charlesworth, 1924) into a lake impounded against a southern ice margin. Lake depth and characteristics would have depended on the interactions of the two contributing ice margins. 4) Formation of Dooneen outwash/delta surface at c. 120m by meltwater from the south. Changes in ice marginal state and thermal characteristics is inferred from evidence of concretions and mud curls within sand facies. Brougher Mountain ice must have retreated into the higher ground at this stage. Lack of evidence for constructional morphologies (i.e. end moraines) may indicate ice stagnation. 5) Formation of the Pubble outwash surface. Variability of palaeodirectional indicators is partly due to the nature of the discrete and enclosed depocentre. It is also related to the ice-distal depositional setting and a likely contribution of water and material by subaerial processes from the adjacent higher ground. Sand and gravel deposition was probably constrained by the position of the easterly ice margin. 6) Formation of the Lough Eyes moraine system. This may be penecontemporaneous with the Pubble outwash surface (cf. McCabe, 1969), with the marginal moraines acting as a buffer between the highly active ice margin and the Pubble outwash which does not show sediment variability, erosional unconformities etc. The moraine system is unusual in that it is so well constrained. The lowland ice mass may have been characterised by two domes: a stagnant dome in the north and an active dome in the south. Also the Topped Mountain massif marks the very end of the western Fintona Hills. This may have acted as an obstruction against ice decay (cf. Knockmany Hill; see Key Site 486 - Clogher Valley). Therefore retreat from this position was probably rapid when the obstruction no longer acted as a pinning point. There is no evidence in the surrounding area for ice stagnation subsequent to retreat from the outer moraines. 7) Meltwater channels trending northeast-southwest along the valley axis incise the sand and gravel surfaces, and contribute to the development of ridge landforms. Channels are up to 100m across and 25m deep. Their size and orientation relative to the sediments they are cut into requires a supporting ice margin on the lowland (eastern) side (cf. McCabe, 1969). Channel continuity through the valley infers that there was one main channelization event after the emplacement of all the sand and gravel areas. A supporting ice margin near to all three areas (Dooneen/Tonyglaskan, Pubble and Lough Eyes) indicates that backwasting did not progress far after retreat from the higher ground. Water was likely contributed throughout the extent of the ice margin, and probably included meltwater from a Brougher Mountain ice mass flowing towards the Pubble area (Charlesworth, 1924). Incision was probably diachronous, and channels dissipate at the southern end of the Lough Eyes moraine, likely due to a loss of ice support, i.e. that Upper Lough Erne was deglaciated at this time (McCabe, 1969). 8) Ice retreat and/or stagnation from these marginal positions into the central Tempo/Clogher valley. 9) Postglacial drainage development. The course of the present-day Tempo River is diverted by the presence of the Lough Eyes moraine. Outflow is now through Maguiresbridge, not Lisbellaw. 10) Postglacial raised bog development and 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: | Diamict, Gravel, Sand |
| Relations: | Ice retreat |
| Geomorph: | deltas, meltwater channel, moraine, outwash |
| Measurements | |||||
|---|---|---|---|---|---|
| Length: | Width: | Height: | |||
| Depth: | Area: | 30km2 | |||
| Access | |
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| Management: | |
This area has generally high morphological intactness. Existing aggregate pits are dominated by finer-grained sediments (medium-grained sands). Specific aesthetic attributes include: 1) Sand and gravel areas are well-constrained, especially around Pubble, and the ridge limits around Lough Eyes. 2) The aesthetic quality of the landscape is generally high. Flat surfaces are maintained around Tonyglaskan and Dooneen. These are accentuated by meltwater incision. Moraine ridges are intact and prominent in the landscape and Lough Eyes is a pleasing landscape resource. 3) Ridge morphology and orientation contrasts with the adjacent diamict ridges which are oriented northwest-southeast. Also the visual and geomorphic contrast between upland bedrock and lowland sand and gravel areas is strong. | |
| Uses | |
| Uses: | Some aggregate quarrying |
| References (11) | |||
|---|---|---|---|
| Boulton, G.S., Caban, P.E. & van Gijssel, K., 1995. Groundwater flow beneath ice sheets: Part I - large scale patterns Quaternary Science Reviews, vol. 14, pp.454-562. | |||
| Carter, R.W.G., Fisher, G.C. & Roberts, J.C., 1977. Carbonate nodules from a lacustrine deposit at Greys Town, County Londonderry Irish Naturalists Journal, vol. 19, pp.4-6. | |||
| 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. | |||
| Hampton, M.A., 1972. The role of debris flow in generating turbidity currents Journal of Sedimentary Petrology, vol. 42, pp.775-793. | |||
| Kehew, A.E. & Teller, J.T., 1994. History of late glacial runoff along the southwestern margin of the Laurentide Ice Sheet Quaternary Science Reviews, vol. 13, pp.859-877. | |||
| Kurimo, H., 1980. Depositional deglacial forms as indicators of different glacial and glaciomarginal en. | |||
| McCabe, A.M., 1969a. The glacial deposits of the Maguiresbridge area, County Fermanagh, Northern Ireland Irish Geography, vol. 6, pp.63-77. | |||
| McRoberts, E.C. & Morgenstern, N.R., 1975. Pore water expulsion during freezing Canadian Geotechnical Journal, vol. 12, pp.130-141. | |||
| Powell, R.D., 1990. Glacimarine processes at grounding-line fans and their growth to ice-contact deltas Special Publications of the Geological Society of London, vol. 53 In Glacimarine Environments: Processes and Products, pp.53-73. | |||
| 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. Tempo Valley Delta Complex - Overview, County Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=485. Accessed on 2026-11-07. |
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