Earth Science Conservation Review
| Fintona Hills Complex - Ballinamallard Esker | Fermanagh, Tyrone |
| Site Type: | Inland exposure |
| Site Status: | TBC |
| Grid Reference: | H244531,H333560 |
| Google maps: | 54.42633,-7.62405 |
| Rocks | |
|---|---|
| Rock Age: | Quaternary |
| Rock Type: | Gravel, Sand |
| Interest | |
| Other interest: | Glacial deposit, esker |
Description:
This area extends from Sidaire (H244531) to Trillick (H333560) (10km) and is characterised by discontinuous, generally straight sand and gravel ridges aligned northeast-southwest. Individual ridges are 100-500m long, 5-10m high and 10-40m wide and separated by breaks 50-100m across. Ridge long axes are generally flat to slightly undulatory. Ridges dip and swell in height and morphology and decline in overall elevation by about 40m from east to west. Only one ridge is usually present at any one location and ridges do not generally bifurcate. Ridges are often contained within a broad (100-150m wide, 10-30m deep) meltwater channel which is cut between diamict-dominated bedforms (200-300m wide, 600-1200m long) aligned northwest-southeast (interpreted as Rogen moraines), and into bedrock.
The ridges are interpreted as subglacial eskers deposited from southwest to northeast during deglaciation under relatively high hydraulic pressure. Evidence supporting this interpretation includes the straight ridge morphology, its clean superimposition upon the diamict bedforms, its uphill climb, lack of slack water-type sand and gravel fan lobes (cf. Gorrell and Shaw, 1991), and its relationship to the Crockfadda outwash-moraine system (Key Site 495). A high hydraulic gradient may be generated by the blocking of subglacial drainage outlets to the west and by changes in glacier morphology caused by its break-up over the Fintona Hills axis. A small exposure near Trillick at (H339558) shows outwash sand, gravel and silt beds recording flow to the northeast.
The Trillick exposure, 500m southeast of the town, is up to 40m long and 2.5m high and probably records the location of a long-destroyed sand pit. There is one main exposure face.
Facies description
This exposure (60m long and 3m high) in a sand and gravel spread shows well-sorted beds (0.3-1.0m thick) of sand, gravel and silt. The central part of the exposure (15m long) is dominated by massive and stratified silts (total 1.5-2.0m thick). Individual layers are laterally continuous, about 1cm thick, and are often deformed. Deformation style includes open and overturned folding (40cm high), with strike alignment approximately northwest-southeast and overturning towards the northeast. The exposure is dominated by dipping and occasionally channelized well-sorted stratified beds (0.3-0.5m thick) of coarse sand, and clast- and matrix-supported granules. Pebble to boulder-sized sandstone clasts (<40cm diameter) also occur as basal lags.
Facies interpretation
These sediments reflect deposition in a proglacial, ice-influenced environment. Evidence for this interpretation includes: the well-sorted nature of the sediments, bedding characteristics, and dipping and channelized contacts. Also the localised well-bedded silts bounded by sand and gravel units, may result from quiet-water deposition, characteristic of ponding in a proglacial outwash environment. Further, ponding may develop between diamict ridges during ice stagnation (Kurimo, 1980). Silt deformation records a strike perpendicular to inferred depositional direction of the sand and gravel facies. This may indicate deformation from glacial overriding of soft, fine-grained sediments, favouring ductile deformation over subglacial shearing and napping. This may indicate (a) that Lower Lough Erne ice had retreated from the Fintona Hills, and (b) that the ice margin was active, which agrees with evidence elsewhere in the Fintona Hills Complex.
Esker ridge sediments are exposed in the Cordromedy exposure (15m long and 1.5m high) 0.6km southwest of Trillick in an esker ridge segment aligned 070-240§.
Facies description
Sediments are dominated by monospecific subrounded sandstone pebble to boulder sized clasts (<50cm diameter). Sediments comprising the core of the esker ridge are planar bedded and composed of clast-supported or openwork pebble gravels with occasional cobbles (<25cm diameter). Sediments comprising the ridge flanks include a bouldery diamict (clasts<30cm diameter) supported by a medium to coarse sand matrix (80%) with bed contacts that dip outwards. Clasts in these flank sediments also dip outwards towards the flanks, with pebble a-axes lying parallel to inferred flow direction (a(p)), and boulder a-axes lying transverse to inferred flow direction (a(t)).
Facies interpretation
The available sedimentary and morphological evidence is consistent with a subglacial esker origin. The monospecific clast lithology, general lack of sedimentary structures and poor sorting favour rapid sedimentation, probably under high velocities. This is also supported by clast a-axis orientation. Flank sediment bedding is interpreted as a primary sedimentary feature. Flow-parallel pebbles and transverse boulder alignment indicate suspension-rafted and bedload transport deposition. Flow support mechanisms include dispersive pressure from the sandy matrix (Hampton,1975). This polymodal mixture was emplaced under high pressure through the discrete esker network. Further, this esker system may have fed the moraines around Crockfadda (Key Site 495) given the high velocity and sediment load of the discharge event(s) resulting in esker facies deposition.
The ridges are interpreted as subglacial eskers deposited from southwest to northeast during deglaciation under relatively high hydraulic pressure. Evidence supporting this interpretation includes the straight ridge morphology, its clean superimposition upon the diamict bedforms, its uphill climb, lack of slack water-type sand and gravel fan lobes (cf. Gorrell and Shaw, 1991), and its relationship to the Crockfadda outwash-moraine system (Key Site 495). A high hydraulic gradient may be generated by the blocking of subglacial drainage outlets to the west and by changes in glacier morphology caused by its break-up over the Fintona Hills axis. A small exposure near Trillick at (H339558) shows outwash sand, gravel and silt beds recording flow to the northeast.
The Trillick exposure, 500m southeast of the town, is up to 40m long and 2.5m high and probably records the location of a long-destroyed sand pit. There is one main exposure face.
Facies description
This exposure (60m long and 3m high) in a sand and gravel spread shows well-sorted beds (0.3-1.0m thick) of sand, gravel and silt. The central part of the exposure (15m long) is dominated by massive and stratified silts (total 1.5-2.0m thick). Individual layers are laterally continuous, about 1cm thick, and are often deformed. Deformation style includes open and overturned folding (40cm high), with strike alignment approximately northwest-southeast and overturning towards the northeast. The exposure is dominated by dipping and occasionally channelized well-sorted stratified beds (0.3-0.5m thick) of coarse sand, and clast- and matrix-supported granules. Pebble to boulder-sized sandstone clasts (<40cm diameter) also occur as basal lags.
Facies interpretation
These sediments reflect deposition in a proglacial, ice-influenced environment. Evidence for this interpretation includes: the well-sorted nature of the sediments, bedding characteristics, and dipping and channelized contacts. Also the localised well-bedded silts bounded by sand and gravel units, may result from quiet-water deposition, characteristic of ponding in a proglacial outwash environment. Further, ponding may develop between diamict ridges during ice stagnation (Kurimo, 1980). Silt deformation records a strike perpendicular to inferred depositional direction of the sand and gravel facies. This may indicate deformation from glacial overriding of soft, fine-grained sediments, favouring ductile deformation over subglacial shearing and napping. This may indicate (a) that Lower Lough Erne ice had retreated from the Fintona Hills, and (b) that the ice margin was active, which agrees with evidence elsewhere in the Fintona Hills Complex.
Esker ridge sediments are exposed in the Cordromedy exposure (15m long and 1.5m high) 0.6km southwest of Trillick in an esker ridge segment aligned 070-240§.
Facies description
Sediments are dominated by monospecific subrounded sandstone pebble to boulder sized clasts (<50cm diameter). Sediments comprising the core of the esker ridge are planar bedded and composed of clast-supported or openwork pebble gravels with occasional cobbles (<25cm diameter). Sediments comprising the ridge flanks include a bouldery diamict (clasts<30cm diameter) supported by a medium to coarse sand matrix (80%) with bed contacts that dip outwards. Clasts in these flank sediments also dip outwards towards the flanks, with pebble a-axes lying parallel to inferred flow direction (a(p)), and boulder a-axes lying transverse to inferred flow direction (a(t)).
Facies interpretation
The available sedimentary and morphological evidence is consistent with a subglacial esker origin. The monospecific clast lithology, general lack of sedimentary structures and poor sorting favour rapid sedimentation, probably under high velocities. This is also supported by clast a-axis orientation. Flank sediment bedding is interpreted as a primary sedimentary feature. Flow-parallel pebbles and transverse boulder alignment indicate suspension-rafted and bedload transport deposition. Flow support mechanisms include dispersive pressure from the sandy matrix (Hampton,1975). This polymodal mixture was emplaced under high pressure through the discrete esker network. Further, this esker system may have fed the moraines around Crockfadda (Key Site 495) given the high velocity and sediment load of the discharge event(s) resulting in esker facies deposition.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Fintona Hills Complex - Ballinamallard Esker, County Fermanagh, Tyrone, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=494. Accessed on 2024-12-26 |
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