ESCR - Ballygawley Delta Complex

Earth Science Conservation Review

Summary Full report
Ballygawley Delta Complex - Overview	Tyrone

Site number:	483
ESCC:	IS
Locality Type:	Inland exposure
Grid Reference:	H613725,H571679 Corners of site, Not Entered
Google maps:	54.59747,-7.05156
County:	Tyrone	Council area:	Fermanagh & Omagh District Council
Period:	Quaternary

Site Description
Highlights:
The Ballygawley delta complex records the deposition of outwash, deltaic and morainic sediments as local ice margins retreated north and south away from the central axis of the Fintona Hills. Flat sand and gravel surfaces around Fallaghearn (H605630) decline northwards into northeast-southwest oriented sand and gravel ridges at Tiroony. The complex is a valuable sand and gravel resource, as evidenced by the number of localised aggregate pits. Deltaic sediments are generally well-sorted and record water flow to the north. Sand and gravel ridges show greater sediment complexity but have little exposure.
Description:
North to south the solid geology of this area can be split into three units: the area around Carrickmore, the Fintona Hills upland, and the lowland area around Ballygawley (Wilson, 1972). These units are separated by major northeast-southwest trending faults of Caledonian age (Ryan et al., 1995). The Carrickmore area is composed of intrusive gabbro, dolerite and epidiorite of the Tyrone Igneous Complex (Hartley, 1933). The Fintona Hills are composed of complexly-faulted Old Red Sandstone and associated conglomerate. These two units are separated by the Tempo-Sixmilecross fault which affected the nature and direction of meltwater erosion (discussed later). The lowland area around Ballygawley is composed of Carboniferous limestones and shales. This unit is separated from the Fintona Hills unit by the Congo Fault. Glacial sediments occur generally below 190m on the north side of the Fintona Hills axis, north of Slievemore and Slievedivena (H580621). These sediments are described here. Those south of the Fintona Hills, i.e. at Ballymackilroy (H625591) are included in Key Site 486 - Clogher Valley. The Ballygawley deltas complex can be split into 3 areas based on morphology, sedimentary characteristics and inferred depositional setting: (1) an area of dissected deltas around Fallaghearn, (2) an area of intact delta and outwash surfaces around Lurganboy (H627650), and (3) an area of aligned ridges around Tiroony. For site specific information see the following sites: Key Site 491 - Ballygawley Delta Complex - Fallaghearn Deltas Key Site 492 - Ballygawley Delta Complex - Lurganboy Delta and Outwash Key Site 493 - Ballygawley Delta Complex - Tiroony Ridges
Importance:
The Ballygawley delta complex is of high importance at the Northern Ireland scale by virtue of its location, extent and morphological characteristics. This is because: A) The delta remnants are part of one of the largest delta complexes in the Fintona Hills, and confirm a south-north direction of water flow. This is contrary to the view of Charlesworth (1924). They also confirm the regional pattern of ice breakup over the Fintona Hills as the first stage of deglaciation. B) The relationship between delta and outwash surfaces show that water depth and location controlled the characteristics of sediment deposition. C) Meltwater channelling is very well preserved in this area, and shows that both bedrock and sand and gravel-incised channels were important at different deglacial stages. D) Two ice margins were involved in the formation and development of the deglacial sediments of this area. The deglacial chronology is rather different to that advocated by Charlesworth (1924).
Interpretation:
This area records the deposition of a variety of sediment facies. Facies type and characteristics were controlled by (a) the relative distance from the ice margin, and (b) the nature of the hydrological setting of the sediment depocentre. This includes water depth and energy. Most sediments are ice-distal. Facies associations, preservation of primary structures and sorting favour more distal environments. Large-scale delta cross-bedding and progradation favour deep-water environments whereas channelised scour and fill facies with variable palaeodirectional indicators favour shallow-water environments. Of particular interest in this area is the pattern and location of meltwater channels. Meltwater channels here are of two general types: those incised into bedrock and those incised into sand and gravel. Bedrock channels usually played an important early role in determining meltwater input and output into impounded glacial lakes into which the sands and gravels were deposited. Channels incised into the sand and gravel spreads were later in time and reflect periods of ice retreat. Incised channels are therefore more topographically defined and often contain present-day misfit streams. Bedrock channels include Todd's Leap and Bernisk Glen. Todd's Leap developed as a subglacial channel carrying water and material northwards to feed the Fallaghearn deltas (Dardis, 1982). Upon deglaciation and the removal of ice from this channel, it served as a southern outlet for impounded water flow to the north, i.e. that flow direction was reversed. The Bernisk Glen channel served as an outflow point of impounded water when it was uncovered during retreat of the northward ice margin (Charlesworth, 1924). It is likely that this channel was subglacially overdeepened prior to deglaciation. Both Todd's Leap and Bernisk Glen show that meltwater used previously-existing topographic lows as lake outflow points, and that the history of lake drainage was strongly controlled by outlet availability. Meltwater channels cut into sand and gravel deposits, especially around Fallaghearn, developed in a southeast-northwest direction, i.e. following present-day topography. This indicates that they developed subaerially. Meltwater channelisation left delta and terrace remnants as interfluves. Eroded sediment was probably redeposited as alluvial fills along these immature valley systems. The following sequence of events in the Ballygawley deltas area is suggested on the basis of the geomorphic and sedimentary evidence outlined above: 1) Late Devensian regional ice flow north to south across the axis of the Fintona Hills. 2) Ice thinning and breakup over the Fintona Hills axis. This resulted in north and south retreat of two ice margins. Breakup was probably to the northern side of the axis, with ice remaining in the higher ground near Todd's Leap (Dardis, 1982). Dardis (1982) also advocates ice lobes in the Owenbrack valley. 3) Delta and outwash deposition. Style of distal sediment deposition was dependent on water depth in the sediment depocentre. Dominance of deltas around Fallaghearn, with outwash surfaces in adjacent and northern positions, indicates that water depth or permanence was greatest nearest to the bedrock upland axis. The transition to outwash sediments shows that decrease in water depth (by ice margin retreat and lake drainage) may not necessarily have been accompanied by a change in the characteristics or location of sediment input. Delta surfaces, at 233-235m (Dardis, 1982) were buttressed by a northern ice margin. 4) Northward ice retreat. Charlesworth (1924) advocates several periods of ice retreat, based on (supposed) ice-marginal moraine formations and the location of lake outlets such as Bernisk Glen. Dardis (1982) also recognises a number of cross-valley ridges. However, in the areas dominated by outwash and delta sediments, moraines are not evident, or may have been altered by meltwater erosion. 5) Oriented ridge (moraine) formation around Tiroony and Gortfin may have resulted from the action of more than one ice margin, especially to the northeast around Tiroony where ice may have been pinned by adjacent high ground. Sand and gravel ridges around Gortfin and Sixmilecross resulted from probably several processes. A high rate of sediment transfer to the margin is inferred from their extent and size. Sediment deposition may also have been partly subaquatic if the ice margin closed off the lower part of the valley. This may account for their ill-defined and rounded morphology. Also, the sand and gravel limit is well-defined to the north and may relate to an ice marginal position. 6) Periods of ice retreat. The northern ice margin may have been characterised by both active and stagnant retreat phases. Active retreat is indicated by a high sediment flux to the margin and moraine formation. Ice stagnation might be inferred from the broad area of lowland peat around Grove Bog (H605705) which contrasts strongly with the moraine morphology to the south. Charlesworth (1924, p.304) believed that no stagnation topography exists in northwestern Ireland apart from around Lough Erne. Retreat of the southern ice margin around Todd's Leap saw ice withdraw through upland cols into the main Clogher Valley. It is likely that this retreat began to take place while water was still impounded around Fallaghearn. This is evidenced by flow reversal through the Todd's Leap channel, and sediment incision to a depth of c. 40m. 7) Meltwater incision. Incision direction is dominantly southeast-northwest through the delta and outwash formations. This post-dates lake drainage (ice retreat). The Sixmilecross-Pomeroy valley is strongly incised by glacial meltwater from northeast to southwest (Charlesworth, 1924 p.265). For this meltwater to cut through the morainic deposits, a buttressing ice margin must have been present to the north to constrain flow direction, at least near the outflow from the higher ground near Tiroony. 8) Postglacial landscape development includes lowland alluviation, mainly by sand and gravel reworking, and terrace cutting. Peatlands also developed across both bedrock and sand and gravel surfaces.
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
Relations:	Deltaic sediments, Glacial deposit, Ice stagnation
Geomorph:	deltas, meltwater channel, moraine, outwash

Measurements
Length:		Width:		Height:
Depth:		Area:	110km2

Access
Management:
This area is characterised by generally high morphological intactness. The major pits at Altagallagy serious despoil the local landscape, however they are fairly well contained and their visual impact is low when seen from the main Ballygawley-Sixmilecross road. Specific aesthetic attributes include: 1) Intactness of the major delta and outwash surfaces. These are enhanced by meltwater incision, which give the landscape impressiveness. Sand and gravel areas contrast with adjacent peat and bedrock areas. 2) The morainic system around Sixmilecross is a major landscape feature. It contrasts in morphology with both the peatlands to the north (Grove Bog) and outwash to the south (Altamooskan). 3) Meltwater channels are also important landscape features, both those incised into bedrock (Todd's Leap, Bernisk Glen), and those incised into sand and gravel (around Fallaghearn). Meltwater channels also contribute to the Sixmilecross moraine.

Access

Management:

This area is characterised by generally high morphological intactness. The major pits at Altagallagy serious despoil the local landscape, however they are fairly well contained and their visual impact is low when seen from the main Ballygawley-Sixmilecross road. Specific aesthetic attributes include: 1) Intactness of the major delta and outwash surfaces. These are enhanced by meltwater incision, which give the landscape impressiveness. Sand and gravel areas contrast with adjacent peat and bedrock areas. 2) The morainic system around Sixmilecross is a major landscape feature. It contrasts in morphology with both the peatlands to the north (Grove Bog) and outwash to the south (Altamooskan). 3) Meltwater channels are also important landscape features, both those incised into bedrock (Todd's Leap, Bernisk Glen), and those incised into sand and gravel (around Fallaghearn). Meltwater channels also contribute to the Sixmilecross moraine.

References (7)
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.
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.
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.
Martini, I.P., 1990. Pleistocene fan deltas in sothern Ontario, Canada Special publications of the International Association of Sedimentologists, vol. 10 In Coarse-grained deltas, pp.281-295 Blackwell, Oxford.
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
Last updated:	23 Dec 2003

Enlander, I., Dempster, M. & Doughty, P., 2026. Ballygawley Delta Complex - Overview, County Tyrone, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=483. Accessed on 2026-11-05.