ESCR - Cumber Delta, Dungiven Basin

Earth Science Conservation Review

Summary Full report
Cumber Delta, Dungiven Basin	Londonderry

Site number:	471
ESCC:	IS
Locality Type:	Inland exposure	Status:	TBC
Grid Reference:	C544074 Approximate
Google maps:	54.91173,-7.15177
County:	Londonderry	Council area:	Derry City & Strabane District Council
Period:	Quaternary

Site Description
Highlights:
The Cumber delta is bounded by the erosive slopes along the present courses of the Glenrandal and Faughan rivers which limit a relatively flat and extensive surface (~126m O.D.) between the townlands of Binn (255700 406100) and Upper Binn (256400 405300). This surface represents the topset surface of the delta. The delta's large-scale foresets record the former presence of an extraglacial lake occupying the Faughan and Foreglen depression as ice retreated generally southwards from the north coast towards the Sperrins. Large-scale lake formation indicates the presence of ice dams blocking the lower reaches of the Faughan and Roe valleys, forming an association of the deposit with the Ballykelly moraine. Locally, kettled topography records the burial of detached ice blocks in the aggraded deposits. The depressions are now occupied by a small (<200m2) raised bog (255500 405800) and Tullintrain Lough (255800 405700). Good exposures occur in commercial quarries at two main locations which combine with the preserved delta topography to give an overall view of the processes which operated during delta formation. The flat-topped delta dominates the skyline topography while travelling east/west along the Faughan River valley, especially near Claudy. The spoil heaps of quarries are visually obtrusive at several sites.
Description:
The Cumber delta consists of two discrete delta fragments, having been dissected along the course of the present day Glenrandal river. It is bordered on its northern and northeastern margins by the River Faughan. Both rivers are now misfit. The main delta surface at Binn (255700 405600) consists of a plateau (1.5km2) of low relief (<5m) lying at 126m O.D. An area of low amplitude hummocky topography occurs at Tullintrain (255800 405300), immediately south and adjoining the plateau. Small-scale hollows in the surface are occupied by raised bogs and lakes. A large-scale depression in the gravels at (255000 405300) contains post-glacial alluvium. The hummocky topography is abruptly truncated on its southern margin by a relatively steep, 6m high slope running parallel to the Longland Road at this location (255900 405100). Further to the south and southeast, approximately 2km2 of gravels are banked against the rising slopes of Slieveboy hill (256000 403300) and extend southeastwards towards Clagan (258200 404800). The exposure logged at Robinson's quarry (253800 406100) shows a sequence consisting of conformably bedded sands (Unit 1) overlain unconformably by pebble and sandy (Unit 2). Unit 1 consists of 1.4m of channelised, conformably bedded coarse sands dipping at a low angle (<5°). The unit contains beds showing both planar-horizontal and rippled lamination. The small scale ripples (<10cm) consist of A-type, B-type and climbing forms. The succession of medium sands is truncated by a clearly defined erosion surface. Above this discontinuity 0.6m of rippled and laminated pebbly sands are interbedded with small-scale (<20cm thick) gravel lags. These are conformably overlain by interbedded trough cross-bedded cobble and ripple laminated sandy gravels. The gravel beds dip at 20° to the north. Fifty meters further south in the quarry approximately 1.3m of flat-lying, poorly-sorted massive cobble gravels occur above an erosive contact with the sandy foresets. The unit has little lateral extent due to incision and removal by post-depositional erosion and quarrying processes. In the western part of the quarry the sandy foresets are overlain by poorly-sorted pebble and cobble gravels. The gravel is contained within multiple channels up to 20m across and 8m in depth. Internally the channel fills consist of stacked beds of variably stratified pebble and cobble gravel which show frequent textural variations. Beds are mainly massive, non-graded and bounded by sharp bed contacts. Beds vary in thickness from 20 to 70 cm. The upper 3m of the sequence contains six large wedges of gravel which truncate the primary gravel bedding. These structures are up to 1.5m across at the top, up to 5m in depth and taper downwards into cracks. A second exposure in the complex occurs in a commercial quarry at Gilky Hill (257100 405300). Several units of matrix-poor, channelised pebble and cobble gravels pass laterally into stacked units of planar laminated and rippled sands contained within channels up to 10m deep and >25m diameter. The channel axes have an orientation of 40°, and dip at <5°. An erosional contact separates both these units from flat lying, channel fills consisting of massive to crudely bedded gravelly sands. Both quarries have a similar pebble-clast composition of 49% schist and 49% vein-quartz, with occasional erratics of quartz porphyry and granites (<1%).
Importance:
Several features of the landform at Cumber are noteworthy in the reconstruction of late glacial depositional environments along the Faughan Basin depression. A. The sediment exposures and flat delta surface are an example of glaciofluvial deltaic formation at the margin of an extensive glacial lake. The large scale channelised foresets incised and poorly-sorted topset gravels exposed at Robinson's quarry illustrates the sedimentological development of a Gilbert type delta. B. The sedimentology of the exposures is clear, demonstrating well-developed proglacial deltaic facies and structure. C. Pristine depositional morphology includes an extensive topographic flat surface at Binn, hummocky gravels at Tullintrain and Alla, kettle holes and possible linear ice-front contact slopes also at Tullintrain. The nature of the sediments and kettling at this location has resulted in an example of raised bog formation in glacigenic topography. D. The sediment body forms an important part of the evidence for testing the theories on the relative chronology of late-glacial events in the Faughan/Dungiven basins. Confirmation that the fragments comprising the Cumber delta are on morphological and sedimentological grounds associated with deposition in a large glacially impounded lake supports Charlesworth's (1924) theory of the former presence of glacial Lake Claudy (cf. Colhoun, 1972). In combination with glaciolacustrine deposits found at higher altitudes elsewhere in upper Faughan and upper Roe valleys, it also provides evidence to support theory that this lake persisted and filled the depressions, as Irish ice progressed southwards. The proglacial lake would thus have expanded to a much greater size than envisaged in these previous models. The implication of variable lake levels is: 1) the continued presence of an ice dam of Irish provenance at Drumahoe; 2) a Scottish ice-dam at Limavady; and 3) its possible continued advance during the late glacial.
Interpretation:
The sediments described consist mainly of large-scale foreset sequences which are associated with deposition in ponded water up to 126m O.D. The well-preserved flat top of the plateau represents a topset gravel sequence which prograded over foresets as water levels shallowed. Palaeocurrent directions of the foreset beds to the north and northwest indicate that ice was retreating generally to the south and east along the western part of the Foreglen depression, into the Sperrins. The general absence of morainic ridges indicates that the deltas were probably fed from subglacial sources emanating from ice-margins grounded in water. The absence of glaciotectonic activity suggests that the ice margins were halting for short periods with no major ice-marginal readvances. The foresets exposed at Robinson's quarry form a typical glaciofluvial delta deposited near a subglacial efflux. The undisturbed regular lamination of the sandy foresets indicates progradation into a proglacial lake. Consistent strikes and dips record sediment input from the south and east. The steeper dipping, coarser foreset beds above the erosion surface represent a significant change in sediment supply regime as the delta prograded or a change in the loci of the subglacial channels. Sediment supply changed to a higher energy, coarser sediment efflux which firstly eroded the existing topsets and foresets before beginning to aggrade a new deltaic wedge. The cobble gravels overlying the sandy foresets are interpreted as topset beds formed in a braided-stream type proglacial environment. Progradation of the sandy foresets occurred after flow separation upon entrance into the proglacial lake. The horizontal surface at Binn (255700 405600) is inferred to be the depositional surface of the deltaic topsets. The area of Tullintrain (255600 404800), Ling (254500 404800) and Alla (253800 404600) immediately to the south of the plateau consists of coarser, sub-aerially deposited gravels. This is shown at an exposure at Ling and from the hummocky topography at Alla. Kettling of the proximal sands and gravels indicates ice block burial beneath aggrading ice proximal sediments. Thus the significant break of slope along the line of the Longland Road (255900 405100) may have been formed against an ice front (in ice contact) and records the position of the ice margin during deposition of the gravel facies farther west at Robinson's quarry (253800 406100). The following model is proposed as the formative event sequence of the area's sediments: 1. As the ice margin retreated south from the Foyle lowlands, a glacial lake developed in the deglacierised area in the lower Faughan valley. To the west it was dammed by a lobe of ice in the Foyle valley, situated approximately at Drumahoe, Co. Londonderry (246000 415000), and to the northeast by Scottish ice at Limavady (267000 423000). 2. Glaciolacustrine sedimentation prograded sandy delta topsets and foresets into the southwest of the lake from an efflux in the direction of the Glenrandal river. Shallow foreset dips (10-20°) represent lower energy sediment gravity flows into the lake. 3. Sediment discharge occurred from at least two major sources, with increased meltwater discharges or delta lobe switching causing localised subaqueous erosion. Stacked delta foreset sequences indicate these slightly different palaeocurrent directions. 4. As water levels shallowed with sediment aggradation, subareal exposure of the sediments resulted in the formation cross-cutting channelised topset gravels. 5. Local stagnation of the ice front resulted in the formation of hummocky gravel topography, the burial of ice blocks and the formation of an ice-contact slope south of Tullintrain. 6. As the ice front retreated southeastwards in the direction of Park (259000 403200) the deltaic deposits exposed at Gilky Hill were formed
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 721 - Cumber.

Keywords
Rocks:	Gravel, Sand
Relations:	Deltaic sediments, Glacio-fluvial sediment, Glaciolacustrine deposit
Geomorph:	braided gravels, deltaic foresets, deltas
Paleoenv:	delta

Measurements
Length:		Width:		Height:
Depth:		Area:	4km2

Access
Management:
Part of the deltaic deposits form an actively exploited aggregate resource of homogenous sands which are at present being excavated especially at Cumber, Templemoyle and Banagher. Extension of quarrying licenses to adjacent areas of the delta bodies may threaten the associations formed on the Tullytrain kettled outwash i.e. the raised bog and Tullytrain lake. A small-scale grassed pit is observable as a result of its 'artificially' steep sides but is not visually obtrusive. The pristine nature of the delta surface is also of conservable quality as an example of this topographic form. The flat surface of the feature is easily observable from main roadways, which pass over it. The intrinsic nature of exposures produced by their partial removal by commercial operations limits their preservation potential. The quarry produces high quality aggregate used in the construction industry and on-site in the production of building blocks. Cessation of aggregate extraction however, while preserving the sediments and topography does not guarantee an extensive lifespan of an exposure due to the rapid degradation of steeply-sloping, scree slopes on high-angle, unconsolidated, highly erodible sediment faces.

Access

Management:

Part of the deltaic deposits form an actively exploited aggregate resource of homogenous sands which are at present being excavated especially at Cumber, Templemoyle and Banagher. Extension of quarrying licenses to adjacent areas of the delta bodies may threaten the associations formed on the Tullytrain kettled outwash i.e. the raised bog and Tullytrain lake. A small-scale grassed pit is observable as a result of its 'artificially' steep sides but is not visually obtrusive. The pristine nature of the delta surface is also of conservable quality as an example of this topographic form. The flat surface of the feature is easily observable from main roadways, which pass over it. The intrinsic nature of exposures produced by their partial removal by commercial operations limits their preservation potential. The quarry produces high quality aggregate used in the construction industry and on-site in the production of building blocks. Cessation of aggregate extraction however, while preserving the sediments and topography does not guarantee an extensive lifespan of an exposure due to the rapid degradation of steeply-sloping, scree slopes on high-angle, unconsolidated, highly erodible sediment faces.

References (2)
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.
Colhoun, E.A., 1972. The deglaciation of the Sperrin Mountains and adjacent areas in counties Tyrone, Londonderry and Donegal, Northern Ireland Proceedings of the Royal Irish Academy, vol. 72 pt. B, pp.91-147.

Record type:	ESCR report
Entered by:	E M Porter

Enlander, I., Dempster, M. & Doughty, P., 2026. Cumber Delta, Dungiven Basin, County Londonderry, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=471. Accessed on 2026-11-06.