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Earth Science Conservation Review
| Mullaghmore Col | Londonderry, Tyrone |
| Site number: | 478 | ||
| ESCC: | IS | ||
| Locality Type: | Inland exposure, Pit | Status: | TBC |
| Grid Reference: | C738008,H684985 Approximate, Not Entered | ||
| Google maps: | 54.84997,-6.85104 | ||
| County: | Londonderry, Tyrone | Council area: | Causeway Coast & Glens Borough Council |
| Period: | Quaternary | ||
| Site Description | |
|---|---|
| Highlights: | |
The steep-sided, probably glacially overdeepened Mullaghmore col (up to 407m O.D.) is a scenic north/south pass through the Sperrin mountain range (locally up to 627m O.D.). The ice-contact glaciofluvial deposits in the col record the retreat of an ice lobe southward towards the lowlands of the Moyola valley. The pattern of deposition indicates that retreat occurred back from the Dungiven basin to the north across the Sperrin mountains towards the lowlands to the south. Glaciolacustrine sequences indicate that small waterbodies were impounded against rising bedrock slopes at the ice-margins during downwastage of the lobe into the col. The intake of the Altnaheglish meltwater channel at the col, at the northern end of which the Murnies delta occurs, indicates that sediment and meltwater for the formation of the delta were probably supplied during the retreat of ice along the col. | |
| Description: | |
The deglacial deposits in the Glenedra col area are described in three main groups, generally from north to south. A Carrickanullar ridges and terraces A series of four northwest/southeast oriented ridges occur on the western margins of the Glenedra col against the rising slopes of Craigagh Hill (271500 398800) at Carrickanullar (271000 399400). From west to east, the ridges decrease in length, depth and elevation towards the valley centre, occurring at approximately 380, 370, 340 and 330m O.D. In the western side of the outermost (highest) ridge a disused sand and gravel pit is utilised as a water treatment works. Small scree accumulations on the ridge slopes and small exposures indicate that the upper portions of the ridge are composed of well-rounded cobble gravel and sand. A palaeoflow measurement taken from a small (<60cm deep) exposure of cross bedded sand and pebbly sand suggests that sediment transport was to 280§N. Along the lower margins of the ridge, where it has been eroded by the Altbritain river, small exposures of parallel laminated silty sand occur. No exposures occur in the other ridges. The four ridges on the western side of the col are separated from a morphologically similar, northwest/southeast oriented ridge on the eastern flank of the col occurring on the lower slopes of Crockalougha (271600 401100) at 340m O.D. by a steep-sided, south/north draining meltwater channel. No exposures occur in the ridge. Glenedra Water terraces From the northwest margins of the ridge complex, a flat-topped, 1km long northwestward sloping plateau occurs between 330 and 320m O.D., 15-20m at most above the present level of the valley floor. The plateau is cross-cut and bisected at 270700 400100 where a disused sand and gravel pit occurs in the northern portion of the deposit. Two smaller terrace fragments occur at 300 and 295m O.D. along the northwestward draining Glenedra Water channel. Altnaheglish Channel The Glenedra channel drains the western flank of the northwest/southeast oriented Crockalougha bedrock ridge, which bisects the col between Mullaghmore and Mullaghaneany. The Black Glen, which also drains northwestwards towards the Dungiven basin, occurs to the east of the Crockalougha watershed. On the northeast margins of Crockalougha a steep-sided bedrock gash intakes at approximately 310m O.D. The channel, which is now occupied by the Altnaheglish river and reservoir, slopes northwestwards towards the Dungiven basin. A rock-cut distributary of the channel terminates at the Murnies Hills glaciolacustrine delta (see Key Site 472 - Murnies Delta, Dungiven Basin). B Labbyheige moraine To the northeast and east of the channel intake, where the col widens out into a 1km2 generally flat-lying area, a complex of up to 4m high hummocks and mounds occurs at up to 360m O.D. at Labbyheige Bridge (272600 402300). At the northern margins of the complex, the steep-sided hummocks occur in a 0.3km long cross-valley (northwest/southeast oriented) linear chain. Immediately south, a less well-defined, generally valley parallel chain of mounds occurs for 0.2km. A disused, completely grassed sand and gravel pit is located in the mounds. No exposure occurs in the complex. Kilcraigagh moraine To the south of both the Black Glen and the Glenedra col, an approximately 0.4 km2 flat-topped to gently undulating cross-valley morainic belt occurs at Kilcraigagh (272800 399400) against the southward facing bedrock slopes. Several poor exposures occur in a disused and almost completely grassed pit at 272800 399200, alongside the B40 road. The sequence exposed consists of massive silty diamict interbedded with beds of poorly-sorted cobble gravel. Immediately east of this site, a small (<1.5m deep) cutting shows highly deformed beds of matrix-poor to openwork pebble gravel. C Meltwater modification Only the deposits at Carrickanullar appear to have undergone significant modification by meltwater channel incision. The Labbyheige deposits occur on the margins of the Black Glen channel axis, away from the Altnaheglish channel, which formed the principal drainage conduit. The Kilcraigagh moraine ridge, deposited against a rising bedrock slope, remained intact due to the backwastage of the ice lobe into the Moyola lowlands. This resulted in the cessation of drainage towards the north and potential incision of the deposit. | |
| Importance: | |
The deposits of the col contribute to the reconstruction of the deglaciation of the main Sperrin mountain range. The deposits demonstrate the active backwastage of ice across the Sperrin highlands southwards into the Moyola valley highlands. The col forms a linkage between the units of the Dungiven basin to the north and Moyola valley to the south. The Altnaheglish meltwater channel, which intakes in the col, forms the sediment supply channel for the high level Murnies delta. Thus the backwastage of an ice lobe in the col was contemporaneous with the presence of a ponded waterbody in the Dungiven basin and the presence of a Scottish ice mass off the north coast. Thus the deglacial features of the col show the relative retreat patterns of the Scottish and Irish Pleistocene ice masses. | |
| Interpretation: | |
The pattern of deglaciation in the area between Mullaghmore and Mullaghaneany is based principally upon the field relationships between deposits and the morphology of the deposits. A lack of exposures prevents comprehensive palaeoenvironmental reconstructions. The ridges at Carrickanullar and across the valley against the slopes of Crockalougha are interpreted as morainic ridges deposited at the margins of an ice lobe occupying the Glenedra col. This interpretation is based on the little sedimentological evidence of glaciofluvially transport (rounded, poorly-sorted clasts) and morphology (steep-sided, linear ridges). If deposition is inferred to have occurred from both englacial and supraglacial sources, then the maximum elevation of the ridges indicates the approximate elevation of the ice margin at that location during deposition. This may have been a result of a period of either increased sediment supply, ice push or relatively stationary ice margins. The decrease in elevation of the ridges towards the valley centre leads to the inference that the ice-lobe underwent backwastage generally southwards and into the centre of the col. Genetically similar deposits are represented by the chain of hummocks at Labbyheige Bridge in the Black Glen, at the eastern side of the col. The cross-valley ridge is inferred to have been deposited at an ice margin fed by south/north flowing subglacial channels, represented by the valley parallel ridge segments occurring immediately southwards. Dissection of these morainic accumulations by northwestward draining meltwater resulted in the formation of the flat-topped glaciofluvial terraces at 320-310m O.D. to the north of the Carrickanullar ridges. The flat-topped, cross-valley belt of material located at Kilcraigagh is inferred to represent ice marginal deposition of sediment into a localised waterbody, impounded by bedrock slopes which rise away from the retreating ice margin. Deposition occurred after the separate tongues occupying the Glenedra col and Black Glen had ablated, resulting in one cross-valley ice margin. The laminated silts located along the lower slopes of the Carrickanullar complex, in the Altbritain valley, are inferred to represent glaciofluvial deposition during late stages of ice retreat southwestwards along the Altbritain valley. The deposition occurred into a low energy environment, possibly a localised waterbody on the glaciofluvial outwash plain. The remainder of the outwash which probably infilled the valley has been removed by meltwater erosion, resulting in the lower elevation glaciofluvial terraces further down the Glenedra channel. SUMMARY The deposits indicate a pattern of generally southward retreat of ice towards the Moyola valley. The occurrence of ice-marginal ridges on both sides of the Glenedra col indicates that ice marginal patterns were strongly controlled by bedrock topography, resulting in the splitting of the ice lobe into two separate glacier tongues, occupying the Glenedra col and Black Glen. Ice pressure resulted in the formation of a separate tongue in the Altbritain valley, which retreated southwestward towards the Glenelly valley. | |
| 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: | Deglacial sediments, Glacio-fluvial sediment |
| Geomorph: | kame terrace, meltwater channel, moraine ridge |
| Access | |
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| Management: | |
The col forms part of the Sperrin mountain range watershed separating the Dungiven basin to the north from the Moyola valley to the south. The elevated location and erosion of the col during the glaciation, provides scenic vistas of the undulating lowland topography to the north and south. The area contains large, mature, managed coniferous plantations e.g. Banagher forest, a managed reservoir (Altnaheglish Reservoir) and wetland vegetation along stream beds, which form a pleasing visual contrast with the large sweeps of reasonably intact heather covered peatland and bare rock surfaces of the rising slopes of the Sperrin mountains. The area contains examples of deglacial landforms and landform assemblages which illustrate deglacial ice-mass dynamics and the relation of landform morphology and sediments to ice mass condition. The ridges and terraces of the area contain several disused sand and gravel pits which remain unvegetated, their scree slopes detracting from the visual appeal of the rugged, barren landscape. | |
| Uses | |
| Uses: | Commercial forestry mainly |
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Mullaghmore Col, County Londonderry, Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=478. Accessed on 2026-11-06. |
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