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Earth Science Conservation Review
| Pomeroy Complex | Tyrone |
| Site number: | 482 | ||
| ESCC: | IS | ||
| Locality Type: | Inland exposure | Status: | TBC |
| Grid Reference: | H26133726,H26823679,H27233663 Corners of site, Not Entered | ||
| Google maps: | 54.28341,-7.59925 | ||
| County: | Tyrone | Council area: | Fermanagh & Omagh District Council |
| Period: | Quaternary | ||
| Site Description | |
|---|---|
| Highlights: | |
The deposits in the valleys around Pomeroy, Co. Tyrone, are located at the southern margins of the Sperrin mountains range and record complex evolution during deglaciation. An ice-sheet which covered this area and the highland areas further north split into two lobes during the final stages of deglaciation upon entering this area during backwastage into the source areas of the Lough Neagh lowlands and the Omagh basin. Ice marginal deposition combined with bedrock topography resulted in the formation of an extensive subaqueous fan, high level Gilbert-type deltas (at 245m O.D.), extensive outwash plains and terraced valley-fills. An example of classic hummocky and ridge topography is located near Pomeroy town. Ice-pressure from the southwest (Omagh basin around Clougher) is indicated by a distinctive sandstone conglomerate contained in the local bedrock, which is traceable as an erratic in the complex and a glacially moulded bedrock knoll to the northwest of Carrickmore town centre which shows ice-flow to 120§N. | |
| Description: | |
The Pomeroy unit contains a wide range of deglacial landforms, including proglacial, paraglacial, subaqueous and subglacial deposits. The area, encompassing 118km2 extends across the Fintona Hills, eastwards from Sixmilecross (257000 367900) to Galbally (272200 365900) and from the southern margin of the Cregganconroe/Evishanoran highlands (265700 375100) to the Owenbrack river valley (263000 365000). The deglacial features are described from west to east from Carrickmore (261300 372600). 1. Ballynapoll fan A 2km2 area of deeply dissected, valley-fill deposits occurs between Aghnaheragh (263600 371600) and Crockroar (265700 371900) along the course of the Camowen River. The deposits are located at the western end of a bedrock depression between Cregganconroe (265700 275200) to the north and the Fintona hills to the south, the western end of which borders the Omagh basin. The fan occurs as two dissected cross-valley ridges. The more extensive eastern ridge at Ballynapoll is deeply dissected by two generally east/west oriented meltwater channels (>40m deep) which merge westwards into one channel. A steep (ice-contact) slope striking northeast/southwest occurs at the northern end of one ridge segment. Ballynapoll exposure The deposit at Ballynapoll consists of two principal LFAs. LFA1-Interbedded sands and boulder gravels: LFA1 is exposed at Ballynapoll at two locations, (264600 371900) and (264600 371400). The exposures (100m long by 25m high each) consist predominately of interbedded, horizontally-bedded silty sands (20%), crudely-stratified to massive polymodal boulder gravels (60%) and cobble to pebble gravels, dipping at low angles (5-15§) to 080§N, with a variance of 120§ in dip direction. Individual gravel beds range between 25cm and 1.5m in thickness. Occasional lenses (1-2m long) of clast-supported cobble-gravel decrease in thickness eastward. The boulder gravel grades eastward into interbedded, tabular, well-sorted, generally <10cm thick beds of sand and cobble-gravel. Post-depositional faults downthrow towards the meltwater channels. Clasts consist predominately (>70%) of fine-grained sandstone and coarse-grained sandstone conglomerate with smaller (<20%) amounts of intrusive basic igneous rocks. LFA2-Rippled, well-sorted sands: 0.35km northwest of an exposure of LFA1, a 35m long by 6m high exposure at (264600 371400) occurs in a terrace at the margin of a 30m deep meltwater channel. No contact between the LFAs is exposed. The section consists of well-sorted, rippled fine-sands occurring in broad (10-15m) channels. Palaeoflows, measured from channel orientation and climbing-ripple orientations were to 230§N. Four west/east trending, linear sharp-crested ridge segments (0.5km long) and an area of hummocky topography occur immediately west and southwest of the deposit's western margin. 2. Tremoge moraine ridges A series of small-scale (<10m high by 15m broad) cross-valley ridges occur in a 2km2 zone from Tremoge (265800 372900) to Rockstown (262300 372700) to the north of the Ballynapoll fan on the rising ground (between 170-190m O.D.) of the Cregganconroe highlands. Several large boulders with a-axes exceeding 3m occur in a concentrated, localised belt on the east facing surface of one ridge at (266400 373200) (cf. Dardis, 1982). One small exposure at (266400 373200) consists of a generally fining upward sequence (8m) of chaotically interbedded massive boulder gravel, openwork pebble gravel and sand. The exposure also contains two large boulders (>1m a-axes) in a sand bed. Palaeoflows, from small-scale cross-bedding structures in sands and apparent bedding dips were to 290§N, varying by up to 180§. 3. Altanagh moraine A 0.5km2 zone of steep-sided, locally 20m high cross-valley and valley-parallel ridges, occurs at the mouth of a small bedrock depression at the eastern margin of the Omagh basin near Altanagh bridge (262400 369900). The valley rises eastward into the Fintona hills, bordered by flat-topped bedrock highs at Inishative and Crockagarran. It contains three 0.25km long, <10m high valley-parallel ridge segments perched on the valley sides. 4. Sluggan outwash A 4km2 area of low relief, dissected valley fill occurs in the valley bottom of a bedrock depression on the northwest margins of the Fintona Hills around Sluggan (265300 370600) immediately southwest of Ballynapoll. No exposures exist in the deposit. 5. Edenfore delta and Owenbrack outwash A flat-topped, dissected, steep-sided deposit is located at the eastern end of a bedrock depression between Cranlome Hill (265900 363900) and Lurganmore (263500 363000). The flat-top is located at 245m O.D. Two large southeast/northwest trending ridges occur immediately to the southwest, forming the high ground at the margins of a large meltwater channel which is oriented parallel to and dissects the ridges and flat-topped deposit. The northwestern margin of the deposit, which contains an overgrown exposure, is bordered by a 3km2 flat-topped, peat-covered plain which is dissected at its western end near Crooked bridge (262900 365400). The deposit is exposed at three locations up to 180m O.D. near its southern margin. The sections consist of interbedded sand, silty sand and pebble to cobble gravel containing small-scale sandy micro-deltas in broad, shallow channels (>20m across by <1m deep). Palaeoflows, from cross-bedding and a-axes imbrication were to 300§N, varying by up to 60§. 6. Pomeroy moraine A 3km2 area of smoothly rolling and locally very hummocky topography occurs between Limehill and Pomeroy Forest (270600 372700) and to the east and southeast of Pomeroy town (269500 372200). The hummocky deposits are best seen in the area of Killey Bridge (270000 371300) where a valley-parallel ridge extends for 3-4km soutwestwards from Pomeroy town. Deposits at Limehill (elevation 200m), immediately north, are banked against the rising slopes of the Cregganconroe highlands. 7. Altmore and Galbally On the eastern flanks of the Fintona hills, flat-topped terraces and dissected deposits occur at Altmore (268200 367900) at 200m O.D. and Galbally (272300 366300) at 110-120m O.D. The Galbally deposits occur in a 1.5km wide zone between streamlined glacigenic ridges oriented south/north and southeast/northwest. | |
| Importance: | |
The complex is rated as having a good scientific importance. This is due to: A. Its demonstration of late-glacial ice retreat directions and thus ice-pressure for ice at the southern margins of the Sperrin mountains-towards the Omagh basin and the Lough Neagh lowlands, B. The evidence for a late-glacial proglacial waterbody at an elevation of 245m O.D. in the deglaciated areas of the Fintona Hills, the south of the Sperrin mountains C. The classic hummocky morainic topography displayed by the Pomeroy moraine D. The evidence of deglacial process in the splitting of the ice mass E. The evidence that the final centres of ice dispersal were in lowland, not upland areas. | |
| Interpretation: | |
The following model is proposed for the deglacial events in the Pomeroy Valley and Fintona Hills area. 1) Ice wastage from highlands to the north: Ice-margin separation During the deglaciation of the Sperrin mountains, the presence of glaciofluvial and glaciolacustrine deposits in the Cregganconroe highlands immediately to the north of the Pomeroy valley indicates the location of a former ice-margin (see Key Site 480 - Ballinderry). Ice-marginal (morainic?) gravels deposited against the northern margins of the valley, form the moraines at Lime Hill and Creggandevesky and indicate a generally southward retreat of the ice-margin in the Pomeroy valley from the highlands to the north. With ice pressure from the west and east together with down- and back- wastage of the ice into the valley the margin split into two lobes, which retreated to the east and the west along the valley. At this time, a localised interlobate waterbody may have formed between the ice-margins and the rising ground to the north, allowing the deposition of the bergstones observed at Tremoge (Dardis, 1982). This location may mark the initial zone of separation. Subsequent westward retreat of the ice-margin towards the Omagh basin is recorded by the trend of the morainic ridges between Tremoge and Rockstown. 2) Westward backwastage: Deposition of the Ballynapoll fan In the Pomeroy valley, between the highland areas of the Fintona Hills and Cregganconroe, a waterbody was impounded against the ice-margins and bedrock (to 170m O.D.). Sediment supply from the west along subglacial and probably ice-marginal conduits (at Altanagh Bridge) resulted in the development of a subaqueous fan at Ballynapoll which prograded eastward (LFA1). The sediment supply route is recorded by the esker ridges immediately west and southwest of the deposit. Later, drainage of the proglacial waterbody westward resulted in the dissection of the fan and the infilling of one channel by reworked sediment at (264600 371400) (LFA2). Reworking of the sediment in the eastern half of the Pomeroy valley resulted in outwash sediments at Crockbrack and glaciofluvial terraces at Pomeroy town. 3) Eastward backwastage: Pomeroy moraine, Fintona Hills deglaciation The eastward retreating ice lobe after depositing the proglacial moraines at Limehill and at Pomeroy forest deposited the well-preserved lateral moraine along the axis of the valley at Pomeroy, from an ice-marginal tongue located in the valley bottom. No exposures are present to confirm this hypothesis. Ice located in the Fintona Hills backwasted up the valleys of these hills from the western margins of the valleys away from the Omagh basin, recorded by a cross-valley morainic complex at Altanagh bridge. After further retreat, valley-fill outwash plains at Owenbrack and Sluggan were deposited. A proglacial waterbody accumulated to a height of 245m O.D. in the deglaciated valleys of the Fintona Hills between ice-margins located at the eastern edge of the Fintona hills and in the Omagh basin (cf. Dardis, 1982). At this time the deltaic deposits at Edenfore (264800 363000) aggraded to this elevation. Large subglacial sediment supplies are recorded by the esker ridges (e.g. at 266900 362100) to the east. Following drainage of the proglacial lake and further retreat of the ice-margin towards Lough Neagh, the flat-topped deltas at Altmore and deposits at Galbally formed in localised waterbodies impounded against the eastern margins of the Fintona Hills. | |
| 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 This site is also a Site of Local Nature Conservation Importance. See Key Site 747 - Pomeroy. | |
| Keywords | |
|---|---|
| Rocks: | Gravel, Sand |
| Relations: | Glacio-fluvial sediment, Glaciolacustrine deposit, Subaqueous outwash |
| Geomorph: | Gilbert-type delta, meltwater channel, moraine, outwash, roche moutonnee, subaqueous fan |
| Measurements | |||||
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| Length: | Width: | Height: | |||
| Depth: | Area: | 118km2 | |||
| Access | |
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| Management: | |
The complex is classed as aesthetically good. This is due to: A. The natural scenic beauty of the Fintona Hills area, B. The aesthetic topographic diversity of the landscape added by the glacial deposits and despite, C). the large-scale commercial sand and gravel quarry visible from the main Pomeroy/Omagh road at Ballynapoll. | |
| Uses | |
| Uses: | Commercial sand and gravel quarrying in parts |
| References (1) | |||
|---|---|---|---|
| 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. | |||
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Pomeroy Complex, County Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=482. Accessed on 2026-11-08. |
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