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Earth Science Conservation Review
| Foyle Valley Complex | Londonderry |
| Site number: | 474 | ||
| ESCC: | IS | ||
| Locality Type: | Inland exposure | Status: | TBC |
| Grid Reference: | C413128,H399858 End(s) of linear site, Not Entered | ||
| Google maps: | 54.96149,-7.35524 | ||
| County: | Londonderry | Council area: | Derry City & Strabane District Council |
| Period: | Quaternary | ||
| Site Description | |
|---|---|
| Highlights: | |
The Foyle valley complex, located along the axes of the Foyle, Mourne, Strule, Glenmornan and Derg river valleys consists of a widespread assemblage of landforms which are genetically linked by formation during ice-margin retreat westward from the Sperrin valleys during the last deglacial cycle. Strong control on ice-margin configuration and meltwater drainage patterns was exercised by bedrock topography, serving to focus meltwater along the valley axes. This resulted in the formation of thick, flat-topped fluvioglacial terraces especially along the Mourne and Strule river valleys. Increases in sediment supply or temporary reductions in ice-margin retreat rates resulted in the accumulation of thick belts of hummocky moraine at Artigarvan and along the Derg river. Most landforms, except for the glaciolacustrine deposits in the Glenmornan valley, are relatively intact and free from commercial sand and gravel extractions. An aesthetically excellent and regionally important landform association is recorded by esker ridge segments, a proglacial moraine ridge and fluvioglacial outwash terrace at Deerpark, 0.5km west of Newtonstewart (239900 385700). | |
| Description: | |
Description of the deposits proceeds from north to south. The complex is divided into two main areas i.e. north and south of Strabane respectively for ease of description. AREA 1: NEWBUILDINGS TO ARTIGARVAN 1. NEW BUILDINGS MORAINE A small belt (<1km2) of hummocky and NW/SE oriented ridge segments occurs along the eastern bank of the River Foyle immediately south of New Buildings (241200 412700) superimposed on small-scale (<3m high) smooth-crested north/south oriented rock ridges. A small (0.75km2) area of flat topography occurs immediately to the east, towards an intake of the Burngibbagh meltwater channel. The latter is up to 10km in length, 60m deep and conducted meltwater northwards into the lower Faughan valley. Northwest of Bready (237400 408100) large-scale north/south oriented smooth-crested ridges are separated from the rising ground of Gortmonly Hill 1.5km to the east by flat, unexposed spreads located at 20m O.D., 10-15m above the level of the alluvium which occurs along the river. Several north/south oriented flutes occur on the side of Gortmonly Hill at approximately 100m O.D. 2. CLOGHCOR MORAINE A deeply kettled area (0.6km2) of hummocky and deeply kettled (>20m deep at Lough Neas) topography occurs to 30m O.D. at Cloughcor (237600 403700) forming high ground between the Foyle and Burndennet valleys at the southern margins of the Burndennet river. The hummocky topography is bordered to the northeast by up to 0.2km2 of flat-topped topography underlain by sand and gravel which slopes northeastwards from 20m O.D. to 15m O.D. Lough Neas exposure A large (0.7km2) partially overgrown commercial sand and gravel pit occurs in the flat-topped topography at (238300 404200). The exposure contains two LFAs. LFA1 consists of up to 5m of large-scale trough cross-bedded sand and pebbly cobble gravel occurring in channels up to 8m wide and 1-2m deep. The sand beds are rippled, containing both sinusoidal and climbing types. Palaeoflow based on channel and ripple orientation was to between 090 and 060§N. LFA2 consists of up to 80cm of massive to crudely-stratified cobble boulder gravel. The exposure is post-dated by a 1m deep ice-wedge pseudomorph. Palaeoflow, based on crude imbrication of cobbles was to 90§N. In both LFAs clastic composition was >85% schist, up to 10% quartz with <5% of coarse grained pink granite. 3. GLENMORNAN VALLEY A belt of flat-topped, steep-sided valley-fill remnants covering 4km2 forms the valley sides of the east/west oriented Glenmornan valley from Grove Hill (238900 399900) to Crockanroe (242900 398600). The deposits are deeply dissected along the valley axis and along the southern valley side by generally south/north oriented meltwater channels. The flat-tops occur up to 160m O.D. along the southern side of the valley, with ridges and mounds occurring along the northern side of the valley to 120m O.D. The flat-tops are intact though several active sand and gravel quarries are located on the lower slopes. Murder Hill exposure An exposure in a large pit on the southern side of the valley occurs between 110 and 130m O.D. and contains two LFAs. The lower LFA consists of 2m of massive to crudely bedded polymodal pebble-boulder gravel with a silty-sand matrix. LFA2 comprises the upper 18-20m of the section, consisting of large-scale planar cross-bedded sand interbedded with pebble to cobble gravel. Occasional beds of massive and normally graded openwork gravel occur. Palaeoflow, based on cross-bedding orientation was to 100§N. Clasts were composed of >80% schist, 5-10% quartz and <5% of coarse-grained pink granite. 4. ARTIGARVAN MORAINE A large (4.5km2) up to 50m thick belt of hummocky topography and cross-valley ridge segments occurs on the eastern margin of the Foyle valley lowlands around Artigarvan (238200 401100) and Ballymagorry (236700 401400). The deposits infill the western exit of the Glenmornan valley to 80m O.D. and form a series of cross-valley ridges from the northeastern edge of Artigarvan up to the intake of the Windyhill channel at 90-100m O.D. The deposits are dissected by an east to west draining meltwater channel network at Artigarvan. The undulating, dissected, eastern margin of the belt at Holly Hill (238400 399900) occurs immediately west of the flat-topped deposits in the Glenmornan valley. No exposures occur in the deposit. AREA 2: STRABANE TO NEWTONSTEWART 1. STRULE/MOURNE VALLEY TERRACES A deeply dissected valley-fill terrace train can be traced along the course of the Mourne river as remnant valley-side fragments from the northern margins of Strabane (234600 397700) in the north to the western margin of Newtonstewart (239900 385700) at Lower Deerpark in the south. From south to north, the terrace level falls from 60m O.D. at Lower Deerpark, to 30m at Mulvin (235800 390100), to 13m O.D. at Drumnabuoy House (234800 395800) to 10m O.D. at Strabane. No exposures occurred in the terraces. To the southeast of Strabane a small (<0.1km2 and 45m thick) flat-topped mound occurs at (235400 396800) at the exit of and partially defined by the Patten's Glen meltwater channel. 2. CASTLEDERG HUMMOCKY MORAINE A large, almost linear 11km long by 1km wide valley-parallel (WSW/ESE) belt of kettled topography, hummocks and valley-parallel ridge segments occurs mainly along the northern side of the Derg river valley from Mulvin Parks (235500 388900) to Castlederg (226300 384700). The belt is continuous and occurs up to 60m O.D. A smaller (2km2), dissected valley-parallel belt of hummocks occurs on the southern side of the valley at Ballynaloan (230700 384200) up to 70m O.D. Several old and now grassed exposures occur along the northern side of the valley and indicate that the hummocks consist of interbedded boulder gravel and sand. 3. DEERPARK MORAINE A sharp-crested, hummocky, steep-sided segmented ridge occupying 1.5km2 occurs to 110m O.D. in a cross-valley arcuate belt from Wood Hills (238000 386000) to Newtonstewart. The ridge has an impressive steep ice-contact southern flank at Laragh. At this site two 3m high ridge segments oriented southwest/northeast (at right angles to the main ridge's orientation) occur approximately 10m from the margins of the deposit and seem to feed into the ridge. The general topography to the southwest of the deposit is flat to gently rolling with occasional small areas of hummocks e.g. at Flushtown (236800 385400). A visually spectacular southwest/northeast sloping, steep-sided meltwater channel up to 20m deep cuts through the deposit at (238300 385800) with channels dissecting the deposit at several other locations along its northern side. The northern flank merges over a smooth, gradual contact into a flat-topped terrace at 60m O.D., which forms part of the Mourne Valley terrace system (see above). 4. MILLBROOK MORAINE A small area (0.4km2) of hummocks and ridge segments up to 20m high occur on both sides of the Mourne river at Millbrook (236500 387700) and occur up to 45m O.D. The Strule/Mourne valley terraces drape the ridge complex to the north and south. Millbrook Exposure A commercial sand and gravel pit exposes 20m of interbedded sand and cobble gravel at Millbrook. The sand units consist of both current bedded and massive beds. The massive to crudely stratified gravel units are up to 5m in thickness. Contacts between beds are generally sharp becoming convoluted in the upper 2m of the section. Palaeoflow, from current bedding in sand was to 330§N. 5. LISATUNNY HOUSE MORAINE A small (0.4km2) area of small hummocks occurs at Lisatunny House (239100 388100) at the intake of a steep-sided south/north sloping meltwater channel (15m deep) which opens to the north into the east/west sloping Douglas Bridge channel. 6. STONEWALLS CHANNEL A 2km long meltwater channel sloping southwest/northeast occurs in the topographic low between Clady Hill (234200 389000) and Whisker Hill (231500 389400). Small mounds and hummocks of sand and gravel define the channel sides on the northwestern side of the valley. | |
| Importance: | |
The Foyle valley complex is rated as having a high scientific importance. This is for three main reasons: A) the unique feature formed by the pristine esker/proglacial moraine/glaciofluvial terrace complex at Deerpark is of national importance as an example of an ice-contact deglacial landform assemblage; B) the Foyle complex demonstrates ice-pressure from the south and retreat of ice-margins into the Omagh basin during the last glacial cycle at the western margins of the Sperrin mountains; C) the morainic belts at Artigarvan and Castlederg form relatively good examples of complex hummocky and ridged (ice-contact) morphology within N. Ireland. | |
| Interpretation: | |
The lack of exposures prevents a detailed palaeoenvironmental reconstruction of the full range of deglacial palaeoenvironments in this area. The general interpretation of the deglacial pattern, especially reconstructed ice-marginal positions in the area is inferred mainly from the relative location of landform assemblages, glaciofluvial terrace distributions and comparison with known patterns of ice wastage in adjacent areas to the east and south. AREA 1: NEW BUILDINGS TO ARTIGARVAN The morainic belts, glaciofluvial and glaciolacustrine spreads occurring in the north of the study area record the retreat of ice margins from at least as far north as New Buildings and from the higher ground of the Sperrin mountains to the east. From field studies of other areas and documented evidence of ice-margins support these interpreted extents. Several north/south oriented streamlined ridges (at New Buildings, Bready and on Gortmonly Hill) support the theory that the Foyle valley depression was used as a conduit for a major ice-stream to the northeast. In the early stages of ice retreat southwards along the Foyle valley, glaciofluvial deposition in contact with decaying masses of stagnant ice located at New Buildings resulted in the formation of hummocky topography superimposed upon northeast oriented ridges which are related to earlier ice-advance directions. An area of flat-topped outwash was also deposited at the intake of the Burngibbagh channel, indicating that the lower reaches of the Faughan channel, into which the Burngibbagh feeds, was ice free at the time. The westward retreat of ice-margins from the Sperrin highlands and valleys into the Foyle valley has been postulated (see Key Site 477 - Lough Fea). Field evidence, such as the valley-parallel ridges in the morainic belt at Artigarvan (rising up to the intake of the meltwater channel at Windy Hill) and the kettled-moraine and outwash complex at Cloughcor which are both banked against the western margins of the Sperrin mountains, support this general theory. Flat-topped, prograded deposits in the Glenmornan valley are interpreted as clastic Gilbert-type deltas and indicate the presence of an ice-marginal waterbody in the valley. The elevation of the flat-top as defined by the subaerial deposition of topset beds suggests that the lake reached an elevation of 160m O.D. The eastward trending palaeoflows recorded in the sediments and proximity of the large morainic belt at Artigarvan indicate that the sediment supply most likely occurred when the main ice-margin was located at Artigarvan. Extensive dissection of the deposits occurred after drainage of the small lake (5km2 in area). AREA 2: STRABANE TO NEWTONSTEWART The general absence of major proglacial morainic deposits in the valley south of Artigarvan until the Deerpark morainic ridge at Newtonstewart indicates that the rate of ice-margin retreat was relatively rapid. The small-scale deposits at Lisatunny House, Millbrook and Stonewalls could be linked in an arcuate band across the Mourne river valley and possibly represent a minor depositional stage as the ice-margin retreated southwards. However they could relate to the locations of three independent ice-margins of slightly different ages. The kettled and hummocky morphology of the deposits in the Derg valley appear to have been deposited over stagnant ice unrelated to ice pressure from the south. The valley-parallel ridge segments in the eastern part indicate that ice retreat was southwards across the valley and not westwards along the valley. The moraine is therefore likely to be partially paraglacial in origin, particularly in its western parts, where valley-parallel ridge segments are absent. The absence of glaciofluvial deposits in the upper parts of the Derg valley west of Castlederg also indicate that ice-pressure was from the south rather than the west. The sharp-crested, linear ridge at Deerpark shows ice-contact slopes on its southern side and associated esker ridges indicating sediment supply from the south. The ridge is inferred to represent an ice-marginal moraine ridge. The esker ridges however do not adjoin the ice-contact slopes directly. The separation is inferred to have been caused by the high (erosive) hydrostatic pressure of the meltwater supply. The main ridge, which has undergone erosion by meltwater after deposition, indicates the location of the ice-margin during the aggradation of the glaciofluvial terraces which begin at its northern margin and occur in a discontinuous northward sloping train as far as Strabane along the Strule and Mourne river valleys. The terraces have been extensively eroded and reworked by northwards flowing meltwater along the Foyle drainage system and are preserved only as valley-side remnants. Post glacial alluvial terraces also occur along the river at lower levels. | |
| 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 | |
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| Rocks: | Gravel, Sand |
| Relations: | Glacial deposit, Glacio-fluvial sediment |
| Geomorph: | flat-topped delta, hummocky moraine, ice-marginal lake, moraine ridge |
| Access | |
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| Management: | |
The Foyle valley complex is classed as excellent aesthetically. This is due to: A) the pristine, visually dominant Deerpark moraine ridge which is partially covered in semi-natural vegetation forms the site for Harry Avery's castle, a well-known local landmark; B) the beautiful landscapes provided by the sweep of the broad, shallow Foyle valley basin infilled by suites of stepped, sculptured glaciofluvial and alluvial terraces seen against the backdrop of the heather covered Sperrin and Donegal mountains; C) the topographic diversity provided by the incised, steep-sided, flat-topped glaciolacustrine deposits in the scenic Glenmornan valley; D) the general lack of major construction or commercial sand and gravel excavations in skyline locations apart from several active pits along the southern slopes of the Glenmornan valley. |
| References (1) | |||
|---|---|---|---|
| Dardis, G.F., McCabe, A.M. & Mitchell, W.I., 1984. Characterisics and origins of lee-side stratification sequences in Late-Pleistocene drumlins, Northern Ireland Earth Surface Processes and Landforms, vol. 9, pp.409-424. | |||
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Last updated: | 6 Jun 2005 | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Foyle Valley Complex, County Londonderry, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=474. Accessed on 2026-11-06. |
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