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Earth Science Conservation Review
| Murrins Complex - Lough Fingrean Frontal Moraines | Tyrone |
| Site number: | 510 | ||
| ESCC: | IS | ||
| Locality Type: | Inland exposure | Status: | TBC |
| Grid Reference: | H573777 End(s) of linear site | ||
| Google maps: | 54.64465,-7.11243 | ||
| County: | Tyrone | Council area: | Fermanagh & Omagh District Council |
| Period: | Quaternary | ||
| Site Description | |
|---|---|
| Description: | |
This area, extending from Lough Fingrean (H573777) to Murrinmaguiggan (4km long and <1.5km wide), is constrained between the bedrock uplands of Mulderg to the northwest and Loughmacrory Hill to the southeast. This area is characterised by 6 major NE-SW oriented sand and gravel ridges (<25m high, 0.2km wide and 1.5km long). Ridges are generally dissected, steep-sided, with flat or undulatory long axes, and are spaced 100-200m apart. Ridges are dominant in the central lowland area where they are an important landscape component, contrasting with the flat outwash surface immediately to the north. Ridges are also associated with adjacent enclosed, water-filled depressions, such as Lough Fingrean and Lough Carn (H574789). The Murrinmaguiggan pit (exposure B) is partly developed in one such ridge (see Key Site 509 - Cashel Outwash). Another ridge section is exposed near Lough Carn (at H582783). The Lough Carn pit cuts at 90§ across the ridge axis and shows a variety of gravel facies. The exposure (<4m high and 20m long) is characterised by sorted clast and matrix-supported granule to cobble gravels (<30cm diameter) in inclined beds (0.3-1.2m thick). The central part of the exposure comprises stacked beds (30-50cm thick; 3m in total) of clast supported and openwork granules and pebbles (<8cm diameter). Clasts are generally subrounded and show a slight down-gradient dip and both a(p) and a(t) fabrics. Beds are tabular in geometry, dip consistently 025§ to 010§ and are separated by diffuse contacts. Occasional outsized boulders (<20cm diameter) are also found. The lowest facies of the sequence is a pebble (<10cm diameter) bed (50cm thick) supported by a well-sorted fine sand matrix (50%). Similar bimodal facies have been observed at many sites. These facies are sharply overlain by a discontinuous (to 4m) massive, vaguely stratified granule bed (30cm thick) with a diffuse upper contact. The top of the exposure comprises a laterally continuous cap (0.4-1.0m thick) of matrix dominant (80%) diamict. Clasts (<8cm diameter) are dispersed and randomly oriented. The exposure is dissected by high angle normal and reverse faults. Strike is consistently 290-110§. Facies interpretation This exposure records a range of events and processes active in an ice-marginal morainic environment. A moraine origin is inferred from the following evidence: (1) ridge morphology and orientation, (2) gravel facies characteristics and dip direction, (3) faulting characteristics, and (4) diamict carapace. Sediment geometry, composition and sorting were largely controlled by the availability of water during sediment deposition. Early stages of sediment deposition were probably subaqueous. This is evidenced by bimodal facies formation by flow partitioning between bedload (clasts) and suspension (matrix). Clast alignment, bed stacking, diffuse bed contacts and matrix deficiency can be explained by bedload carriage and fine-sediment winnowing by active water flow. Bed dip is a primary characteristic, indicating a relatively steep hydraulic gradient and ice-marginal proximity. A high-energy ice margin often produces chaotic boulder beds with erosional scouring (Rust and Romanelli, 1975). That chaotic boulder beds and scours do not characterise this exposure and may be due to: (1) its proposed subaqueous setting, which would dissipate the effect of a point-source sediment input, (2) the association of frontal ridges with feeder eskers (Dardis, 1982), i.e. that any high-energy boulder facies are likely to have been removed from the sediment system, and (3) the extent of the ridge system in the Lough Carn area. This infers that moraines were built up by more widespread sediment conveyance to the margin, possibly by subglacial shearing and the addition of englacial debris. This point is discussed later. Fault orientation is approximately parallel to that of the ridge long axis, and number and magnitude of faults decline south to north. Faulting may therefore be explained by ice activity to the south of the ridge. Orientation consistency of high angle faulting was generated by a loss of sediment support as ice retreated south (Dardis, 1982). Normal and reverse faults accommodate the vertical and lateral changes necessary to bring about morphological stability. Diamict carapace development post-dates faulting. Clasts in the diamict and gravel components are of similar size, shape and lithology, which suggests a similar source and transport mechanism. Ice marginal shearing of subglacial material may elevate debris in the profile. This material may be released by ice melt and added to the moraine by slumping and gravity flow processes. This diamict is therefore interpreted as a debris flow deposited from the ice front during ice stagnation or retreat. This interpretation of moraine ridge formation differs from that of Dardis (1982) who believed that moraines were fed by discontinuous northeast-southwest trending eskers feeding in from the southwest. Further, Dardis (1982) related moraine ridges to lake levels, and interpreted moraines as composite ice-marginal fans. | |
| Notes: | |
For site specific information on other sites within the Murrins Complex see the following sites: Key Site 508 - Murrins Complex - Glensawisk Burn Delta Key Site 509 - Murrins Complex - Cashel Outwash Key Site 511 - Murrins Complex - Laght Hill Retreat Moraines Key Site 512 - Murrins Complex - Coolesker/Beragh Eskers For overview information and references on the Murrins Complex see the following site: Key Site 479 - Murrins Complex - Overview 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 |
| Geomorph: | moraine ridge |
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Murrins Complex - Lough Fingrean Frontal Moraines, County Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=510. Accessed on 2026-11-07. |
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