Glenelly 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 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 Glenelly Valley is often regarded as one of the most idyllic in Northern Ireland. The Glenelly River flows westwards along a long linear glen to the south of Sawel Mountain, following one of the principal fault-lines in the Sperrins. The verdant valley landscape contrasts with the expansive, windswept moorland above and has a hidden, secretive character. The valley itself is relatively narrow and enclosed by steep ridges. The slopes are gently undulating and divided into pastures and oak woodlands by stone walls and hedgerows. Tree cover increases towards the valley floor, where the Glenelly River meanders across a complex, undulating floodplain of alluvium, glacial moraine and glaciofluvial sands and gravels that record ice margin retreat generally west and south towards the Foyle/Strule valleys and the Omagh basin. The channel has often carved deep ridges within these deposits, creating steep, irregular mounds and pockets of peaty marsh on the valley floor. Deeply incised meltwater channels cut in rock and drift are also numerous, for example at Barnes gap. The wide, alluvial bottomlands form strong visual contrasts with the steep, erosional scars forming the margins of the present river channels and the peat covered surrounding highlands. The landscape is highly valued for its heritage and scenic beauty and is designated as part of the Sperrins AONB. It is an extremely sensitive landscape with significant archaeological sites and a historic field pattern. In this narrow valley, the detailed composition of the landscape pattern is visually important and very sensitive to change, as views are contained and relatively short from one side of the valley to the other.

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)

This LCA is dominated by Dalradian (Neoproterozoic) strata of the Sperrin Mountains. These rocks were originally sediments and igneous rocks that have been buried and metamorphosed. The igneous rocks are now metabasites (oriented NE-SW), exposed in the stream of Glensass Burn (ESCR Site 317). The Garvagh Bridge Member (a limestone) of crops out in ESCR Site 319). The Glenelly Formation and one member (Clogherny) are also seen in the crags and small quarries of Barnes Gap (ESCR Site 320).

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 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 valley that is largely underlain by till that was deposited by the Late Midlandian passage of ice up the valley from an ice centre to the west. However, the Quaternary features that are of greatest geomorphological and geological significance are the deglacial sands and gravels left as the ice retreated. These deposits are represented by the Glenelly Valley Deglacial Complex. Glaciolacustrine spreads and cross-valley morainic ridges record westward retreat of an ice tongue along the Glenelly River valley with consequent ponding of a proglacial lake against the surrounding bedrock slopes. Valley-side subaqueous fan sediments may reflect meltwater deposition from ice margins retreating to both the north into the Sperrins and to the south across a bedrock ridge. The Glenelly River valley complex is visually intact and is located in a highly scenic area adjacent to the Sperrins. Sand and gravel extraction is limited to small-scale pits reflecting local use of the resource. Much of the aggregate on the higher valley-sides is in any case buried under overlying unsorted material, or diamict, the result of post-depositional reworking. The B47 is way-marked as a Scenic Route along the length of the valley and the Ulster Way skirts the southern fringe of the complex. Overlapping glaciofluvial terraces, fan-deltas and moraine ridges contrast with larger scale landforms immediately to the north and south. The Glenelly River itself is generally untouched and the valley floor contains examples of abandoned meanders. Minor elements of the complex occur downstream in LCA 27 and upstream in LCA 24.

Key Elements

Deglacial Complexes

GLENELLY RIVER VALLEY complex

The area is of importance in understanding the complexity of deglacial processes late in the deglaciation of Northern Ireland, as ice thinned and was constrained by topographic barriers between 17,000 and 13,000 years ago. The glaciofluvial valley fill of the Glenelly River valley is generally intact, with little fluvial reworking following evacuation of the valley by ice.

Other sites/units identified in the Earth Science Conservation Review

317 Glensass Burn

Precambrian. Southern Highlands Group. Stratotype of Henry's Bridge Member of Dart Formation.

319 Garvagh Burn

Precambrian. Glenelly Formation. Stratotype for Garvagh Bridge member. Limestone unit in turbidite facies rocks.39

320 Barnes Gap

Precambrian. Deeply incised glacial overflow channel, containing good outcrop of Glenelly Member.

321 Barnes Burn

Precambrian. Type strata of Clogherny Member - distinctive series of calcareous schists and psammites. Access to Glenelly Formation.

AONB

Lies completely within the Sp[errin AONB (1968). This designation is indicative of the scenic quality of the landscape.