Foyle Valley 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 Western River Basins. This region consists essentially of the connected river systems that drain the Carboniferous and Old Red Sandstone plateau of County Tyrone, as well as the foothills of the Sperrin Mountains to the east and Donegal to the west. The region extends from the Omagh Basin in the south, northwards along the lower Foyle valley. The Omagh Basin has particular significance as an ice centre during the Late Midlandian and is now largely covered by a complex mixture of glaciofluvial sands and gravels and drumlins overlying Rogen moraines. When the headwaters of these river systems rise together they have in the past been responsible for serious flooding at the bottleneck of Strabane. Although this has been mitigated by extensive drainage control works in and around the town.
The Foyle Valley follows the border with the Republic of Ireland to the south of Londonderry, before turning eastwards at Strabane to follow the meandering course of the river (known as the Mourne and, to the south of the confluence with the Derg, the Strule). It is steeply enclosed to the east by the slopes of the hills to the west of the Sperrins range. It continues to the east of Newtownstewart and includes the lower reaches of the Owenkillew River. The character of the river channel varies from an open sheet of water between agricultural fields to the north of Ballymagorry, to an incised, wooded channel to the south of Strabane. The river flows within a deeper valley in areas where it is influenced by glacial moraine. The steep, irregular mounds of moraine on the banks of the Strule to the west of Newtownstewart are a distinctive local landmark. There are arable fields as well as pastures in areas with a shallower land form and in the Maghereagh area, where there is an alluvial plain alongside the Foyle. By contrast, the tributary valleys of the Burn Dennet (near Milltown Burn Dennet) and the Glenmoran River (by Artigarvan) to the west of this flat valley floor, have a deeply undulating, secretive character. Key elements in the landscape are the extensive glaciofluvial landforms that consist of thick, dissected mounds and spreads of morainic and outwash deposits, forming large-scale undulating and hummocky belts, sharp-crested ridges and flat-topped valley-floor terraces. The margins of the valleys bounding the deposits are bedrock hills up to 150 - 200m O.D. and the rising ground of the Sperrin Mountains to the east (up to 400m O.D.) and streamlined hills in Co. Donegal to the west. Exposures in the deposits are rare.
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 |
|---|
Owenkillew |
Dalradian (Neoproterozoic) - about 600 million years old |
Undifferentiated Southern Highland Group |
Un-named metabasites |
Londonderry |
Ballykelly |
Dart - Claudy |
Dungiven - Aghyaran |
Newtownstewart |
This LCA is dominated by Dalradian (Neoproterozoic) strata of the County Londonderry succession. Lateral equivalents from the Donegal and Tyrone succession occur in the area. These rocks were originally sediments and volcanic rocks that have subsequently been buried and metamorphosed. Pillow Lavas of the Dungiven Formation are exposed in the Glenmoran River (ESCR Site 329). Ballykelly Formation granular, pebbly tourmaline schists with some original hydraulic sedimentary structures occur in the southeastern edge of LCA27. Exposed in Kittybane Quarry (ESCR Site 326).
Londonderry Formation tourmaline schists and green beds with limestones occur in the northern strip of LCA27. Exposed in Prehen Quarry (ESRC Site 337).
Two tectonic phases have affected the area: the Caledonian (Ordovician - Silurian) and Variscan (end Carboniferous).
The ESCR Site of 328: Strabane Quarry, occurs in this LCA.
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.
The drift geology map for this LCA shows a landscape that is largely underlain by Late Midlandian till. This was laid down by ice that flowed northwards and eastwards along the present-day Foyle valley from a centre in the Omagh Basin to the south. However, from a geomorphological and geological standpoint, the Quternaru deposits of greatest interest are the deglacial sands and gravels that were laid down as the ice wasted and the ice margin retreated southwestwards. This LCA contains elements of four such deglacial complexes that are important scientifically and for their sand and gravel resources. However it is dominated by the Foyle Valley Complex.
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, 0.5 km west of Newtonstewart . This LCA contains 16.5km2 of this complex out of a total of 31km2. Other deposits are found in LCAs 20, 26, 29, 30 and 31.
The Dunnamanagh Complex occurs as a small area in the east of the LCA (1.3km2). It comprises glaciofluvial landforms that 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. Most of the complex occurs in LCA 30, where it is described in more detail.
The Faughan/Dungiven Basins Glaciofluvial Complex occupies a very small area of outwash (0.8km2) at the very northern end of the LCA. The principally deltaic deposits of the complex occur mainly in LCA 30 and are described in more detail there.
The Glenelly Valley Complex occurs as a series of moraine ridges in the extreme southeast of the LCA. Most of the complex is in LCA 28, where it is described in more detail.
Key Elements
ASSI
DEER PARK MORAINE AND OUTWASH (NEWTOWNSTEWART)
A massive sharp-crested moraine ridge extends westwards from Newtonstewart and merges on its northern flank with a flat-topped terrace that lies along the western bank of the Strule River. Two esker ridge fragments are located to the south of the main ridge. Landforms mark a major ice stillstand during final retreat towards the Omagh Basin. Steep ice contact slopes and deeply entrenched meltwater channels produce dramatic landscapes of high aesthetic quality.
Deglacial Complexes
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. It also shows, through the development of northward sloping glaciofluvial terraces that the mouth of the Foyle valley was ice free as ice retreated southwards. Finally, an esker, moraine ridge and outwash valley fill complex at Newtonstewart forms an important example of a deglacial landform association.
ARTIGARVAN MORAINES (STRABANE)
These moraines form part of the Foyle Valley Complex and 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
328 Strabane Quarry
Precambrian. Pillow lavas in Dalradian succession on northern limb of Sperrin Overfold.
336 Kittybane Quarry
Precambrian. Quality outcrop of Ballykelly Formation on south of Lough Foyle Syncline. Preserved sedimentary structures.
337 Prehen Quarry
Precambrian. Exposure of turbidite metasedimentary rocks of Londonderry Formation, on southern limb of Lough Foyle Syncline.
329 Glenmornan River
Precambrian. Argyll Group. Early Irish examples of Dalradian pillow lavas. Stratigraphic marker horizon in Dungiven Formation.
AONB
Small areas of the Sperrin AONB (1968) lie in the south and west of the LCA. This designation is indicative of the scenic quality of the landscape in this area.
