| Previous Site | Next Site |
Earth Science Conservation Review
| Murnies Delta, Dungiven Basin | Londonderry |
| Site number: | 472 | ||
| ESCC: | IS | ||
| Locality Type: | Inland exposure, Pit | Status: | TBC |
| Grid Reference: | C6706 | ||
| Google maps: | 54.89763,-6.95568 | ||
| County: | Londonderry | Council area: | Causeway Coast & Glens Borough Council |
| Period: | Quaternary | ||
| Stages: | Late Glacial | ||
| Site Description | |
|---|---|
| Highlights: | |
The Murnies delta is an example of a highly disrupted landscape in a skyline setting. The sand has been extensively quarried over two decades for its mineral aggregate value, removing most of the centre of the delta and obscuring lateral slopes with quarry waste. Bedding planes exposed through the commercial workings show stratified sands and gravels dipping northeastwards in the direction of the Benady Glen and the lowlands of the Dungiven Basin. Remaining frontal slopes are steeply dipping (30§), sub-parallel to exposed foresets, indicating a non-ice contact depositional origin. The sediment source was an ice lobe located in a col to the southwest of the site which supplied meltwater along the Altnaheglish channel. Deltaic geometry indicates deposition of a Gilbert type delta into a ponded water body with surfaces at 262 and 243m O.D. The mechanism of the water body's impoundment is hypothesised to be ice damming in the coastal lowlands to the north and northwest. The delta represents highly important evidence for the ponding of an extensive waterbody in the area to the north of the Sperrins range during the late-glacial. | |
| Description: | |
A 0.75km2, wedge-shaped, two-tiered landform is situated at 240-270m a.s.l. on the northward facing, blanket bog covered slopes of Teeavan Hill, approximately 3km southwest of Dungiven, Co. Londonderry. The site occupies a skyline position in the Benady Glen, visible from along a major east/west route through the Sperrin mountains (A6 Londonderry/Belfast road). The delta's apex lies at the northeastern opening of the rock-cut Altnasheskin channel (cf. Colhoun, 1972) located in the col between the Teeavan and Tamniarin Hills. The landform is divided morphologically into two bi-lobate flat-topped levels, bordered laterally by two meltwater channels. The upper level is located at an altitude of 262m O.D., the lower at 243m O.D. The tiers are unequal in volume, the lower, chronologically younger level, considerably larger. Each level is bissected by a meltwater channel which is occupied by the White Burn, a small (<2m across) tributary of the River Roe. Dissection has divided each delta level into two approximately equal portions. The delta has been the site of a commercial quarrying operation for some time. Much of the original flat, deltaic morphology and internal bedding structure of each lower lobe have been destroyed. A small area (100m2) of the lower tier's flat-top remains, with a 2-3m deep covering of blanket bog. Both high angle (>25§ dip) northwest/southeast striking foreslope-parallel frontal slopes and lower angle (<15§) northeast/southwest striking lateral slopes remain partially (<60%) visible, except where obscured by ungrassed spoil-heaps. The landform consists of stratified sand and gravel overlain by a drape of diamict. Exposed sediments consist of large-scale planar cross-bedded well-sorted medium-grained sands. Clasts consist mainly of rounded quartz (98%) and schist (1%) pebbles. | |
| Importance: | |
The delta is classed as highly scientifically important. Its high altitude (262m a.s.l.) location on the southern slopes of the Dungiven basin provides important information about events during the deglaciation along the Sperrins northern foothills. It indicates the existence of an extensive, high-level, ponded water body in the Dungiven depression during the deglaciation of the region. The water level controlling upper deposition surfaces is also shown to have dropped by almost 20m during the time of the delta's formation. This represents a relatively rapid, catastrophic drainage of an extensive water body. It is an excellent, topographically discrete example of a prograded, coarse-grained glaciolacustrine delta which was dissected during a slight fall in water level. | |
| Interpretation: | |
The sand and gravel deposit is interpreted as a glaciolacustrine delta which prograded into a standing water body located in the present day Benady Glen area. The meltwater source was located to the southwest, in the uplands of the Sperrin mountains providing sediment via the Altnasheskin channel. Large scale foresets show delta progradation to the north and northwest. The overlying diamict drape probably represents mass flow of sediment associated with a fall in water level. The presence of ponded water in Benady Glen can be explained by two main hypotheses: A) Colhoun (1971) envisaged a localised origin for the standing water, formed at the southwestern margin of an ice-mass which occupied the Benady Glen. Therefore, the steep frontal slopes of the delta are inferred to be ice-contact in origin. B) The alternative view is that a large body of water dammed against the foothills of the northern Sperrin range. The sedimentary sequence of the deposits, from the available sedimentological evidence, indicates that the feature is a Gilbert-type delta, with steep frontal slopes sub-parallel to foreset dips. The frontal slopes of the delta are thus primary depositional features and not ice-contact in origin. This hypothesis envisages deltaic formation in a much more extensive lake dammed to the north by Foyle Valley ice at Drumahoe and ice of Scottish provenance at Limavady. In addition, evidence for basin-wide, high-level deltaic sedimentation occurs in the Dungiven depression and eastwards to Feeny. Regional evidence in the form of coarse-grained deltas along the northern foothills of the Sperrins supports the view that the widespread ponding occurred. Evidence in the form of a moraine across the lower Roe Valley indicates that Scottish ice was the agent responsible for water ponding at a variety of levels from 119m to 262 m. | |
| 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: | Diamict, Gravel, Sand |
| Relations: | Glaciolacustrine deposit |
| Geomorph: | Gilbert-type delta, proglacial lake |
| Measurements | |||||
|---|---|---|---|---|---|
| Length: | Width: | Height: | |||
| Depth: | Area: | 0.75km2 | |||
| Access | |
|---|---|
| Management: | |
The landform is located on a rural hillside in a scenic, mountainous area (on the northern flanks of the Sperrin AONB). Its high altitude location and formation in the late-glacial period (both reducing the probability of post-depositional meltwater erosion) provided good conditions for its preservation as a pristine example of a classical Gilbert-type delta i.e. flat topset units and steeply dipping (foreset controlled) foreslopes. The vast majority of the interior of the landform (>60%) has however been destroyed by a commercial quarrying operation. Most of the upper tier remains, as well as a small portion of the lower delta including the high angle foreslopes which are in places covered in spoil (>40%). Quarrying operations at the time of writing have been halted, and the site closed to public access. Removal of the flat delta-tops has resulted in the destruction of a 3m deep (as evidenced by the remaining portion) blanket bog. The drainage system of the remaining peat bog has now however been fundamentally altered. The ecosystem supports a rich flora with a substantial population of bog-cotton. A temporary, unmetalled road cuts across the mountain blanket bog covering Tamniarin Hill and has resulted in disruption of drainage, accelerated roadside gully erosion and a young conifer plantation. The stark contrast of the roadway and the sand of the ungrassed spoil to the hillside vegetation coverage can be observed to the south of the A6 Londonderry to Belfast road for several kilometres to the east of Dungiven. A 0.15km2 portion of the original flat-topped deltaic morphology has been restored at a level 30-40m below the original surface. Further restoration of deltaic morphology is thought to be feasible at this site. The rejuvenation of the mire ecosystem is also a long-term possibility. | |
| Uses | |
| Uses: | Quarrying operations have stopped and the site has no public access. |
| References (2) | |||
|---|---|---|---|
| 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. | |||
| McCabe, A.M., 1994. Sand and Gravel landscapes in Northern Ireland - a scientific basis for conservation Earth Heritage, vol. 2, pp.18-22. | |||
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Murnies Delta, Dungiven Basin, County Londonderry, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=472. Accessed on 2026-11-06. |
| Previous Site | Next Site |