Sperrin Foothills 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 Foothills lie to the north and west of the Sperrins and include the rounded summits of Slievekirk (370m), Eglish (277m), the Highland Hill complex, Crookdooish (321m) and the river valley systems of Burn Dennet and the Faughan River. Fast-flowing streams have eroded deep, winding valleys, carving the landscape into rounded, dome-shaped hills. It is a dynamic landscape, with neat, rolling farmland, steep, wooded valleys and rounded 'caps' of moorland on the summits of the higher hills. Glacigenic and glaciofluvial deposits often form complex, steeply undulating landforms on valley sides. Loughs and areas of peaty marsh, such as Lough Ash, are occasionally found in poorly drained hollows. The west of the LCA has a high aesthetic value because of the visually attractive, deeply dissected glaciofluvial terraced infill occurring along the axis of the Burn Dennet river valley and the girdle of heather covered Sperrin mountains which form a hilly backdrop to the better managed land along the axis of the Dunnamanagh basin. The deep, steep-sided, often wooded, meltwater channels cut into the deposits infilling the upland valleys provide abrupt and visually pleasing topographic contrasts to the gentle gradients of the surrounding mountains.
Sand and gravel pits are common on the edges of the valleys, where glaciofluvial deposits provides an available source, and there are many examples of quarries that form visual scars and have devastated the valley landscape such as near to Fawney, on the NE bank of the Burn Dennet. Key elements in the landscape include:
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)
Carboniferous - about 350 million years old |
|---|
Barony Glen |
Dalradian (Neoproterozoic) - about 600 million years old |
Un-named metabasites |
Ballykelly |
Dart - Claudy |
Dungiven - Aghyaran |
Newtownstewart |
This LCA is dominated by Dalradian (Neoproterozoic) strata of the Sperrins succession. Structural strike is dominantly east-west to northeast - southwest (NE-SW) with an anticline in the southwest and an overall northerly dip in the north. Metamorphosed limestones, basaltic pillow lavas are exposed with quartzites in the pits and crags of ESCR Site 315 at Craig. Sedimentary and structural features are well exposed at Kildoag Quarry (ESCR Site 332); the Alla Limestone Member is exposed at Drain Quarry (ESCR Site 331).
The Ballykelly Formation - Comprises granular, pebbly tourmaline schists in the northern tip of LCA30. Exposed in Tamnymore Wood (ESCR Site 334).
Two tectonic phases have affected the area: the Caledonian (Ordovician - Silurian) and Variscan (end Carboniferous). Caledonian deformation is very apparent throughout the Dalradian succession.
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 period. 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.
This LCA contains some of the largest expanses of Late Midlandian glaciofluvial sands and gravels in Northern Ireland. These are to be found in the valleys of a series of rivers that drain the Sperrin Mountains and flow northwest to join the Foyle. In the south west of the LCA, The Dunnamanagh Complex mainly occupies the structural low of the Dunnamanaghgh basin / Burn Dennet river valley (11,7km2). In the centre of the LCA, the Faughan valley contains significant elements of the Faughan/Dungiven Basins Complex. A small area of this complex also occurs in the northeast of the LCA in the Roe valley. Together, these two areas cover 15.5km2. Finally, in the southwest of the LCA there is a small area of the Foyle Valley Complex (2.29km2) including the Artigarvan Delta shared with LCA27.
The Dunnamanagh Complex comprises a range of Fluvioglacial landforms consist of thick, dissected accumulations of morainic, outwash and glaciolacustrine deposits, forming large-scale undulating belts, flat-topped valley-floor terraces, hummocky topography and spectacular examples of later meltwater incision. Sediment supply was from ice margins retreating generally south and southwest from the Faughan valley towards the Sperrin uplands to the south and Foyle valley to the west. A small area of this Complex also occurs in LCA 27.
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. The high relief range allows pleasant views both from the basin bottoms and from the Sperrin valleys. There is a general lack of commercial sand and gravel production in the area except immediately east of Dunnamanagh and another, larger pit at Moyagh. Smaller areas of the Complex occur in LCAs 27, 29, 31, 33, 34 and 37.
The Foyle Valley Complex consists of a widespread assemblage of landforms that 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 glaciofluvial terraces. 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.
Key Elements
ASSI
061 ERVEY WOOD
Woodland with structural diversity and variation ranging from calcifugous to strongly flushed. Physical features include a number of small waterfalls and wet rock faces, as well as a series of high cliffs and a broad river flood plain.
Deglacial Complexes
The Dunnamanagh Complex
The area is of importance due to the large scale recessional moraine belts which record relative ice marginal positions during ice retreat and the control of upland bedrock topography on the ice-margin decay pattern of a decaying ice sheet. It demonstrates the deglacial sequence following the ice retreat from the Faughan Valley southwards and the spatial relationships between morainic belts and glaciofluvial outwash.
the Faughan/Dungiven Basins Complex
Deltaic deposits are preserved at seven principal locations and are grouped within three main altitudinal levels, representing the control of former ice-dammed lake levels on their formation. They 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.
the Foyle Valley Complex
The complex has a high scientific value, for understanding the complexity of deglacial processes and records ice retreat westward from the western Sperrin valleys into the topographic low of the Foyle valley, indicating ice pressure from the direction of the Omagh basin to the south during the last deglacial cycle. Most of this Complex is in LCA 27, smaller areas in LCAs 20, 21, 26, 29 and 31.
Cumber delta (Faughan/Dungiven basin complex)
An extensive area (approx. 4 km2) of glaciolacustrine deltaic sand and gravel and hummocky gravel overlain by proglacial gravel which partially fills the Faughan valley bedrock depression immediately south of Claudy, Co. Londonderry. It is a site of excellent scientific importance and is a pristine example of deltaic morphology. demonstrating a late glacial proglacial lake at 126m O.D.. It also demonstrates proglacial sedimentological processes, ice frontal position and the direction of ice retreat in this region southwards into the Sperrin Mountains.
LISNARAGH MORAINE AND OUTWASH (Dunnamanagh Complex)
This is a well-marked ridge across the Burn Dennet valley fronted immediately to the north by a valley-side glaciofluvial terrace. The Lisnaragh moraine is a discrete, but representative, example of the morainic landforms that were deposited during deglaciation in this area. The landform association marks a stage in the southward retreat of ice towards the Sperrin Mountains late in the deglacial cycle. The Lisnaragh moraine/outwash couplet is an integral part of an area of high aesthetic value. Pristine morainic ridges and deeply incised streams combine to produce high relief within a relatively low altitude range.
AGHABRACK OUTWASH (DUNNAMANAGH COMPLEX)
A strikingly flat surface to the west of the Burn Dennet lies behind and is separated from a hummocky moraine ridge by the meltwater-eroded trench of the river valley. Together with an esker ridge alongside a minor tributary of the Burn Dennet these features are an excellent example of a deglacial landform assemblage. Incision by meltwater has produced steep slopes, particularly along the margins of the Burn Dennet channel, resulting in considerable landscape interest. This is a largely unspoilt area on the northern edge of the Sperrins, and grades gradually into the lower mountain slopes.
Artigarvan moraines and outwash (Foyle valley Complex)
These moraines are a notable example of large scale moraine building in a valley setting that are of importance in understanding the complexity of deglacial processes during the recent glacial history of Northern Ireland. A well-marked belt of contiguous rounded ridges, piled against one another across the entrance of the Glenmornan River valley, marks halts in the retreat of ice from the western Sperrins into the Foyle valley. Geomorphologically, this is a pristine area, and landforms have been little disturbed by human activity. The rolling landscape, with steep ice contact and ice distal slopes, and deeply incised meltwater channels, has considerable aesthetic appeal.
Other sites/units identified in the Earth Science Conservation Review
315 Craig
Precambrian. Dungiven Formation. Relatively undeformed basalt pillow lavas. Limestone and quartzite closely associated.
332 Kildoag Quarry
Precambrian. Southern Highland Group. Quality outcrops of coarse-grained arenaceous rocks of Claudy Formation. Sedimentary and structural features well displayed.
334 Tamnymore Wood
Precambrian. Good exposure of lowermost stata of Ballykelly Formation.
331 Drain Quarry
Precambrian. Claudy formation. Lithology and structure of Alla Limestone Member. Good exposure and access.
AONB
The southern margin of the LCA lies in the Sperrin AONB (1968). This designation is indicative of the scenic quality of the landscape.
