ESCR - Derrylin Complex

Earth Science Conservation Review

Summary Full report
Derrylin Complex	Fermanagh

Site number:	487
ESCC:	IS
Locality Type:	Inland exposure	Status:	TBC
Grid Reference:	H22353226,H22723187,H22813276 Corners of site, Not Entered
Google maps:	54.23868,-7.65797
County:	Fermanagh	Council area:	Fermanagh & Omagh District Council
Period:	Quaternary

Site Description
Highlights:
The Derrylin complex forms the most extensive and thickest glaciofluvial complex in southwest Northern Ireland and the western Lough Erne basin. The complex is located between an extensive drumlin swarm to the east and a bedrock high to the west. Sediment exposures allow a detailed reconstruction of the palaeoenvironment, including the glaciotectonic deformation of the sediments.
Description:
The Derrylin complex occurs in a 6km long by 2km wide at most linear belt along the eastern footslopes of Slieve Rushen mountain, Co. Fermanagh, 6-7km west of Upper Lough Erne at the southwestern margin of the Lough Erne lowlands. The complex is described from south to north, and has been divided into three principal morphological units. 1. Steep sided ridges Three steep-sided linear ridges ranging from 30m high and 70m wide, to 4m high and 15m wide oriented generally southeast/northwest are located to the south of Gortineedan (228100 321200) between the drumlin belt occupying the Erne basin lowlands and the rising of Slieve Rushen. The ridges rise to the north and northwest, terminating in the southern and southeastern margins of the steep sided belt of sediment around Gortmullan (227500 322100). The largest ridge extends southwards into Co. Cavan, Republic of Ireland. Several abandoned, grassed pits are located in the generally flat to undulating topography, which occurs for up to 150m around the ridges. 2. Steep sided bank A continuous belt of sediment is banked against the westward rising sandstone slopes of Slieve Rushen (223400 322600) from Gortmullan in the south to Camletter (227400 325900) in the north. The flat to undulating topped belt is connected to three steep sided ridges at its southern and southeastern margins, has a steep eastern flank, and appears to decrease in thickness northwards. Several exposures occur in the belt. Gortmullan exposure: A large, composite exposure (>300m long by 25m deep) in the thickest part of the sequence at Gortmullen shows three principal LFAs. LFA1 consists of poorly-sorted, chaotically-bedded, sandy pebble to cobble gravel. Thickening upward units of vertical beds containing clusters of vertically oriented clasts occur through the LFA. LFA2 consists of interbedded sand, pebbly sand and cobble gravel in stacked channels up to 40m wide by 10m deep. Southwest/northeast striking shear zones (rising to the northwest) and asymmetric folds, with steeper northern fold limbs, occur along the length of the exposure. LFA3 occurs above a sharp, planar contact, consisting of up to 1.5m of massive cobble gravel. The composite section shows that the sediment generally fines to the northwest. A largely grassed, predominately disused pit occurs at (227200 323800) 1.5km further north along the belt at approximately the same elevation. A 7m long by 3m high exposure at the crest of the deposit shows a sequence of well-sorted sand beds in a 5m wide by at least 2.5m deep channel. The upper surface of the channel has been truncated by a sharp contact with cross-bedded, well-sorted cobble gravel. 3. Ridges, low mounds and hummocks Several localised patches of mounds, ridge segments and kettle holes occur at Derrylea (227700 325300) and Knowley Rectory (227800 323800) along the eastern margins of the main flat-topped belt. The most extensive belt occurs from Callowhill (227800 326600) to the eastern edge of Derrylin (228100 327600), at the northern extreme of the complex. The topography consists of a 1.5km long sinuous belt of ridge segments, hummocks and mounds of low local relief (up to 3m).
Importance:
The high scientific importance of the Derrylin complex is principally related to the information the sedimentology and glaciotectonic deformation of the sequence provides about the ice-marginal configuration in this area of the province during the last deglacial period. The inferences drawn of ice-pressure to the south and southwest mean that an ice divide was located in this direction, towards the central lowlands of Ireland. Such north/south decay vectors also indicate a probable south to north ice advance direction, as supported by the roche moutonn‚e of Knockninny Hill and the regional pattern of drumlinisation as a result of ice downdraw north and northwestwards onto the continental shelf via Donegal Bay (Eyles and Mc Cabe, 1989).
Interpretation:
The following interpretation of deglacial events in the Derrylin area, at the southwestern corner of the Lough Erne basin, is based on the morphological and sedimentological evidence recorded for the glaciofluvial complex together with the patterns of glacigenic erosional features (e.g. drumlins, roche moutonn‚es) in the surrounding region. The thickest deposits occur as a steep-sided bank of sediment against the rising bedrock of Slieve Rushen in a region otherwise poor in glaciofluvial sediment accumulations of any size. The juxtaposition of the smaller, linear, steep-sided ridges and Erne basin drumlin field indicates possible relations between the landforms. The steep-sided morphology and sedimentology of the deposits in the Derrylin complex seem to indicate that the accumulation of sediment occurred in ice-contact locations. The steep-sided ridges at the southern and eastern margins of the deposit, which rise to the northwest, in the direction of the thickest sediment accumulations, probably represent subglacial conduits which transported sediment towards an ice margin. The margin of the ice-mass under which they flowed appears to be indicated by the linear, ice-contact slope defining the eastern edge of the main ridge. The sedimentology of the main ridge indicates that the majority of the glaciotectonically thickened pile aggraded in a subareal ice-contact environment, probably within a lateral embayment in an ice margin located along the eastern footslopes of the Slieve Rushen massif. The low mounds and ridge segments to the north of the main ridge around Callowhill and Derrylin may represent ice-marginal morainic accumulations or the northern margins of the main sediment belt. The vertical and lateral geometry of the lithofacies associations and juxtaposition of ice-contact landforms (steep-sided ridges, kettle-holes) conforms to classic models of ice-marginal glaciofluvial deposition. The glaciotectonic deformation of the outwash sediments is inferred to have resulted from an ice-mass margin fluctuation and compressional compaction of the sediment pile. From the strike (120/290°) of the shears and asymmetry of the fold limbs which verge towards the northwest, it is inferred that ice-pressure was from the south and southeast. The morphological and sedimentological evidence indicates therefore that glaciofluvial outwash sediment accumulated in an ice-contact situation, along the eastern margins of the Slieve Rushen massif during the deglaciation of the area. Steep-sided linear ridges to the south and southeast probably represent subglacial esker sedimentation in conduits draining generally northwards from a zone of ice dispersion to the south. Similar evidence of ice-pressure to the south is represented by the geometry of glaciotectonic deformation in the sediments. The glacigenic landforms in the Derrylin complex are associated with ice pressure from the south and southwest, indicating the presence of a generally east/west ice divide in that direction. The drumlin swarm to the east of the complex in the Lough Erne lowlands are therefore probably similarly related to ice flow from the south and southwest, towards the north and northeast. In this light, the 'crag and tail' morphology of Knockninny Hill (227500 330500) (Chapman, 1970), located 2km north of Derrylin, is probably a large roche moutonn‚e, moulded by ice advancing from the south (cf. Sugden, Glasser and Clapperton, 1992). The preservation of the Derrylin complex in close proximity to the drumlin swarm indicates that the complex post-dates the drumlinisation process. The ice-marginal fluctuation recorded by the deformation of the sediments therefore indicates that the ice-mass remained active after the drumlin forming event.
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:	Gravel, Sand
Relations:	Glacio-fluvial sediment, Glaciotectonic
Geomorph:	drumlin

Access
Management:
The Derrylin complex has been the location of extensive and intensive commercial aggregate production, with the landscape now a concentrated zone of disused and active pits. Old pits pockmark the entire belt, with many left derelict and the sites of ad hoc dumping. A large, visually dominant cement producing plant is located at the southern end of the main belt. Dust pollution from the works blankets the area and vehicular traffic from the plant seems to far outweigh the intended carrying capacity of the local routeways. The area is otherwise one of high natural beauty, and a popular routeway through and to the Fermanagh Lakelands. The industry associated with the aggregate supply and cement production are the only detractions from scenic qualities of the area.

Access

Management:

The Derrylin complex has been the location of extensive and intensive commercial aggregate production, with the landscape now a concentrated zone of disused and active pits. Old pits pockmark the entire belt, with many left derelict and the sites of ad hoc dumping. A large, visually dominant cement producing plant is located at the southern end of the main belt. Dust pollution from the works blankets the area and vehicular traffic from the plant seems to far outweigh the intended carrying capacity of the local routeways. The area is otherwise one of high natural beauty, and a popular routeway through and to the Fermanagh Lakelands. The industry associated with the aggregate supply and cement production are the only detractions from scenic qualities of the area.

References (2)
Chapman, R.J., 1970. The Late-Weichselian Glaciations of the Erne Basin Irish Geography, vol. 6, pp.153-161.
Sugden, D.E., Glasser, N. & Clapperton, C.M., 1992. Evolution of large roche Moutonnees Geografiska Annaler, vol. 74 pt. A numb. 2-3, pp.253-264.

Record type:	ESCR report
Entered by:	E M Porter

Enlander, I., Dempster, M. & Doughty, P., 2026. Derrylin Complex, County Fermanagh, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=487. Accessed on 2026-11-06.