Sperrin Mountains Geodiversity Profile
Outline Geomorphology and Landscape Setting
The use of a cultural overlay in defining Landscape Character Areas (LCAs) means that they frequently subdivide natural physiographic units. It is common therefore for significant geomorphological features to run across more than one LCA. It is also possible in turn, to group physiographic units into a smaller number of natural regions. These regions invariably reflect underlying geological, topographic and, often, visual continuities between their component physiographic units, and have generally formed the basis for defining landscape areas such as AONBs. It is essential therefore, that in considering the 'Geodiversity' of an individual LCA, regard should be given to adjacent LCAs and to the larger regions within which they sit. In the original Land Utilisation Survey of Northern Ireland, Symons (1962) identified twelve such natural regions.
This LCA lies within the region described as the North Derry Uplands and Sperrin Mountains. This region has a composite geological structure. In the north, the North Derry Plateau is wholly developed on basalt and defined by a steep, unstable escarpment to the west and a set of structural benches dipping gently to the east. Southwest of this plateau land, and beyond the Glenshane Pass, schists and quartzites form the rounded, whaleback ridges of the High and Low Sperrins. The incised, steep-sided valleys of rivers such as the Glennelly and Owenkillew accentuate the southwestwards, Caledonian structural trend of the Mountains. Late Glacial ice recession from around the mountains and the creation of temporary ice-dammed lakes has left valley floors and slope foot zones mantled in thick, complex glaciofluvial deposits. Northwest of the Sperrins is a dissected block of country underlain by schists that forms the Loughermore-Altahullion hills and the Middle Faughan basin.
The Sperrin Mountains form a spine across the North West and a backdrop to views. Formed from resistant metamorphic Dark Schists and Upper Glenelly Schists, they extend broadly east - west across the southern part of County Londonderry and the northern part of County Tyrone. The steeper summits average 500m, with the highest peak, Sawel Mountain, rising to 678m. The Sperrins have a dramatic, mountainous appearance; the ridges have a broad, rounded profile, leading to summits with a rocky, pointed silhouette. Glacial deposits sometimes form mounds and terraces along the lower slopes, softening and confusing the natural beak of slope. Fast-flowing moorland streams are fairly straight and open in character, sometimes eroding deep channels between steep ridges of glacial moraine. Deep gullies, some with broken, eroded edges, create strong dendritic patterns, carving and moulding the steep slopes into striking forms. Outcrops of grey rock and minor screes litter many of the slopes. Water flows in deep gullies to the upland plateau. Summits have extensive areas of bog supporting acres of heather and rushes. The bogs are punctuated by small, rounded loughs, the source of many streams. Although the mountains were overridden by ice during the Midlandian, studie of deposits associated with the deglaciation of the region suggests that permafrost and severe periglacial conditions continued into the Holocene following the retreat of the ice. As a consequence, Colhoun has identified a range of periglacial phenomena, including: ice wedge pseudomorphs, cryoturbation structures, fractured pebbles, soliflucted materials, talus deposits, rotational slumps and slump talus, blockfields and stone polygons.
This open mountain landscape is extremely sensitive to change since even relatively small elements in the landscape, such as electricity pylons or a single building are visible over long distances. The most significant pressures for change are from conifer plantations, mineral extraction and windfarms on the upper slopes, and built development on the valley slopes. The upland summits are generally in good condition, but the quality of the landscape deteriorates towards the edges of the character area, particularly on the lower summits to the west, where transmission masts, roads and conifer plantations have a cumulative negative influence. The Sperrins have long been the source of building materials and are now subject to exploration for gold. The quarries form prominent scars on the landscape, visible for miles around. The plant, machinery and roads associated with the works are also a negative visual influence. There is very little evidence of new buildings on these upland slopes. The landscape can therefore be summarised as one of broad, rounded ridges with rocky outcrops leading to steep, pointed summits, deep, branching gullies and open, fast-flowing moorland streams.
Pre-Quaternary (Solid) Geology
The stratigraphy of this area is made up of the mapped formations in the table, the youngest of which usually overlie the oldest. The older formations can be upside down (tectonically inverted).
Stratigraphic Table (youngest rocks at the top of the table)
Tertiary - Upper Basalt Formation - about 55 million years old |
|---|
Triassic - Sherwood Sandstone Group - about 240 million years old |
Carboniferous - about 350 million years old |
Barony Glen |
Owenkilllew |
Dalradian (Neoproterozoic) - about 600 million years old |
Un-named metabasites |
Glenelly |
Dart |
Dungiven |
Newtownstewart |
This LCA is dominated by Dalradian (Neoproterozoic) strata of the Sperrin Mountains. These are metamorphosed sedimentary and igneous rocks, as seen in the Newtownstewart Formation in Letterbrat Quarry (ESCR Site 326) and the Dungiven Formation (interbedded dark biotite schists with thin limestones) in the pit at Balix Hill (ESCR Site 330).
The ASSIs of ESCR Sites 314 and 316 of Butterlope Glen and Mullaghcarbatagh Mountain and ESCR Site Oughtboy Burn occur in this LCA.
Owenkillew Sandstone Group - occupies a 2km by 3km area in the southwest of LCA29 where the pebbly sandstones rest unconformably upon Argyll Group Dalradian. Supposed to be the lateral equivalent to the Iniscairn Sandstone, these rocks can be seen in unconformable contact with Dalradian in Spincha Burn (ESCR Site 254). Barony Glen (ESCR Site 256) also encroaches into this LCA.
Caledonian tectonic fabrics dominate the rock succession which is folded and faulted.
Quaternary (Drift) Geology
Northern Ireland has experienced repeated glaciations during the Pleistocene period that produced vast amounts of debris to form the glacigenic deposits that cover more than 90% of the landscape. Their present morphology was shaped principally during the last glacial cycle (the Midlandian), with subsequent modification throughout the post-glacial Holocene epoch. The Late Midlandian, the last main phases of ice sheet flow, occurred between 23 and 13ka B.P. from dispersion centres in the Lough Neagh Basin, the Omagh Basin and Lower Lough Erne/Donegal. The clearest imprint of these ice flows are flow transverse rogen moraines and flow parallel drumlin swarms which developed across thick covers of till, mostly below 150m O.D. during a period that referred to as the Drumlin Readvance. At the very end of the Midlandian, Scottish ice moved southwards and overrode parts of the north coast. Evidence for deglaciation of the landscape is found in features formed between the glacial maximum to the onset of the present warm stage from 17 and 13ka B.P. - a period of gradual climatic improvement. Most commonly these are of glaciofluvial and glaciolacustrine origin and include: eskers, outwash mounds and spreads, proglacial lacustrine deposits, kame terraces, kettle holes and meltwater channels (McCarron et al. 2002). During the Holocene, marine, fluvial, aeolian and mass movement processes, combined with human activities and climate and sea-level fluctuations, have modified the appearance of the landscape. The landforms and associated deposits derived from all of these processes are essentially fossil. Once damaged or destroyed they cannot be replaced since the processes or process combinations that created them no longer exist. They therefore represent a finite scientific and economic resource and are a notable determinant of landscape character.
The efficiency of glacial erosion Over much of this mountain area means that it carries little in the way of glacial and glaciofluvial deposits. However, there are limited glacio flucial deposits (1.8km2) in the headwaters of the Roe and Faughan Rivers that form part of the Faughan and Dungiven basins Complex. In the west of the LCA there is a small element (1.0km2) of the Artigarvan Moraines and Outwash assemblage that lies mostly in LCA27. The Drift Geology mapfor the LCA also shows that as elevation decreases to the west, the landscape takes on a cover of Late Midlandian till laid down by ice that was centred on the Omagh Basin. Finally, The east of the LCA contains the full extent (1.9km2) of the The Mullaghmore Col Glaciofluvial Complex.
The Mullaghmore Col Glaciofluvial Complex is an An assemblage of ice-marginal morainic ridges and glaciofluvial terrace fragments that occurs between 450 and 300 m O.D. in the rock cut col between the glacially eroded, largely drift free slopes of Mullaghmore and Mullaghaneany mountains. The ice-contact glaciofluvial deposits in the col record the retreat of an ice lobe southwards 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 Faughan/Dungiven Basins Complex consists of glaciofluvial deposits that are primarily deltaic in origin and are situated along structural lows in the upper Faughan and upper Roe drainage basins. The area is of high scientific interest due to the presence of extensive glaciolacustrine and glaciofluvial deposits consisting of deltas, moraines, eskers and outwash plains occurring in close field associations. Most of the Complex occurs in LCA 30, smaller areas occur in LCAs 27, 33, 34, 31 and 37. In this LCA the 1.8km2 of deposits centre on the Tamnagh Eskers and valley fill in the headwaters of the Faughan.
Key Elements
ASSI
064 BANAGHER GLEN (ca 50% shared with LCA 37)
Largest serving intact block of semi-natural woodland in NI and the finest example of calcifuge oak woodland found. It has an important exposure of Upper Dalradian orthotectonic series, including the Dungiven Limestones , limited extrusive epidiorite and the Dart schistose grits. The major Sperrin fault passes through the site. The site is a complex system of river valleys. The underlying geology gives rise to a wide range of growing conditions ranging from strongly calcifuge types through to flushed calcicolous types.
Deglacial Complexes
The Mullaghmore Col Glaciofluvial Complex
The Mullaghmore col assemblage assists in the understanding the recent glacial history of Northern Ireland through deposits that record the retreat of an ice lobe located in the col southwards towards the Moyola valley from the Dungiven basin to the north. The southward retreat pattern indicates that ice pressure during the last glacial period across the Sperrin mountains from the direction of the Moyola and Glenelly valleys. The landforms provide topographic variety to an otherwise monotonous forest and heather covered highland landscape. However, deposits have been quarried in several locations, forming scars in the otherwise continuous cover of heather and forest.
The Faughan/Dungiven Basins Complex
Deltaic deposits are preserved at seven principal locations and are of special scientific interest, as their widespread extent and relationship to proglacial water levels implies that substantial, deep lakes were impounded along the Faughan and upper Roe valleys as Irish ice masses retreated southwards and Scottish ice advanced southwestwards into the lower Roe valley. The upper Roe (Dungiven) and middle to upper Faughan valley basins have been used for mineral aggregate production in the northwest of the province for approximately twenty years.
TAMNAGH ESKERS & VALLEY FILL (Faughan/Dungiven Basins Complex)
Cross-valley ridges and sediment wedges cross the valley of the Sluggada Burn, some 12km to the southwest of Feeny. Up-valley, three distinct steep-sided ridges cross the valley and trend towards the lower landforms. These are interpreted as eskers feeding sediment into the valley during southward ice retreat through the Sperrins. Steep slopes and well-marked, relatively small-scale landforms result in a varied, attractive, landscape within a well-defined valley setting.
MURNIES DELTA TEEAVAN (Faughan/Dungiven Basins Complex)
A glaciolacustrine deltaic complex is perched on the side of Teeavan Hill, on the northern flanks of the Sperrin range. The delta is two tiered and extensively quarried for its mineral aggregate reserves. The landform has excellent scientific merit and can be explained by the presence of an ice dam of Scottish provenance in the coastal lowlands of Lough Foyle during the late-glacial. This provides important regional chronostratigraphic evidence for late-glacial events. Intensive quarrying has destroyed much of its glacigenic morphology. This feature overlaps the boundary between LCAs 29 and 37.
Other sites/units identified in the Earth Science Conservation Review
314 Butterlope Glen
Precambrian. Good exposures of lithology and structural elements of Dungiven Formation. Lower strata of Dart Formation.
316 Mullaghcarbatagh Mountain
Precambrian. Access to exposures of lowermost stratigraphic levels of Dart Formation.
318 Oughtboy Burn
Precambrian. Glenelly Formation. Stratotype for the Chloritic green bed Oughtboy Burn Member.
326 Letterbrat Quarry
Precambrian. Argyll Group. Exposures of Newtownstewart Formation (oldest rocks in Sperrins). Undeformed porphyry intrusion.
354 Spincha Burn
Carboniferous. Exposure of Iniscarn Sandstone Formation resting on Dalradian basement.
256 Barony Glen
Carboniferous. Exposure of Drumard Member strata. Evidence of soil formations in some limestone bands.
330 Balix Hill
Precambrian. Good exposures of one limestone member within the Dungiven Formation. Shows lithology and structural relationships in the limestone.
AONB
Lies completely within the Sperrin AONB (1968). This designation is indicative of the scenic quality of the landscape.
