Slieve Russel, Derrylin and Kinawley 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 Plateau and Valley Lands of Fermanagh. This is a series of sharply defined plateau blocks separated by steep-sided, glacially deepened, lake strewn valleys. The morphology of the Carboniferous uplands is largely controlled by the presence of a series of gritstone caprocks. Beneath these, well-jointed limestones have allowed the development of extensive subterranean drainage systems as well as a variety of surface karst phenomena for which the region is internationally renown.

This LCA is a complex landscape unit on the southern boundary of Fermanagh. It has contrasting elements of upland and lowland landscape. The area is dominated by Slieve Rushen, a small flat-topped isolated block of limestone, sandstone and shales, which rises to 403m. Its steep slopes are dissected by small glens that run down to complex glacial deposits on the lower slopes. The two major summits of Slieve Rushen and Molly Mountain are separated by the Owengarr River. The area also includes the drumlin lowlands and lowland bog to the east and the glacial trough occupied by the Cladagh River. The summit of the mountain is covered by blanket bog, which has been disturbed by peat cutting and erosion. The slopes of the uplands are quarried for limestone and sand and gravels are extracted and processed in the Gortmullan area, giving this area an industrialised and degraded character. Key elements in the landscape therefore include: steep-sided upland rising above low drumlin hills. A continuous belt of glaciotectonised glaciofluvial deposits occur as a bank of sediment for 7km along the eastern footslopes of Slieve Rushen from south of the Northern Ireland border at Ballyconnell, County Cavan to Derrylin.

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.

Stratigraphic Table (youngest rocks at the top of the table)

This LCA is comprised of fossiliferous Carboniferous sedimentary rocks south of Upper Lough Erne with the exception of the Tertiary dolerite dyke. The Carboniferous formations occur as arcuate east-west and northwest-southeast striking, discontinuous outcrops.

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. 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 the area clearly identifies the drift free upland of Slieve Rushen, although glacial striae and streamlined rock ridges on its northwestern margin do indicate that it was overridden by ice that flowed approximately southwards across the area. This view is confirmed on the lowlands to the east of Slieve Rushen, where Late Midlandian till has been shaped into numerous drumlins aligned approximately N-S. Drumlins are also found to the north of the uplands, but their orientation is less decisive and has probably been influenced by local topography.

Within Northern Ireland drumlins take a variety of forms; some are rounded in plan, although the majority are elongated in the direction of ice flow. Some have sharp crests, whereas others are more whaleback in profile. Although most drumlins are composed of glacial till or tills, a small number are 'drumlinoid features' are rock-cored and some are composed of sand and gravel. Where drumlins are rock cored there may have been significant frost shattering prior to their shaping by ice flow. It is possible therefore to see tails of shattered debris within till leading away from the feature in the direction of flow (Davies and Stephens 1978). It is generally accepted that the drumlins of Northern Ireland were formed by deposition beneath fast flowing ice. In the majority of cases this has resulted in a thick layer of Upper (younger) Till overlying a core of Lower (older) Till. This pattern has been observed across Northern Ireland, apart from a limited area in the north of County Down. The precise temporal relationship between the two tills has not been definitively resolved, but Davies and Stephens (1978) refer to an organic layer between the tills in County Fermanagh that has been dated at 30 500 ± 1170/1030 years B.P. and shelly material between the tills on the Ards Peninsula dated at 24 050 ± 650 years B.P. However, these deposits only indicate that the Lower Till is older than the dates obtained.

It can be argued that an equally important component of any 'drumlin landscape' are the similarly numerous inter-drumlin hollows. The majority of these hollows would have held open water from local runoff at the end of the Pleistocene. Whilst some continue to exist as isolated small loughs, many have now been infilled by sediment washing off the surrounding drumlins. This has created typically flat-bottomed, marshy areas between the drumlins that are subject to seasonal inundation. Much of the infilling probably occurred early in the Holocene, as the landscape adjusted to increasingly temperate conditions. However, erosion may also have been accelerated in historical times, when rural population densities were considerably higher and much of the lowland landscape of Northern Ireland was more intensively cultivated. Whatever the stimulus for erosion and deposition, the sediments within these hollows typically contain an important record of local environmental change. Within this LCA, the Drift geology map shows that many of the inter-drumlin hollows in the east and north of the LCA have been infilled by lacustrine alluvium associated with post-glacial flooding of the landscape and the creation of Upper Lough Erne.

Before the inundation of the Lough Erne basin occurred there is evidence that there was extensive fluvioglacial activity and deposition on the flanks of Slieve Rushen during the deglaciation of the area. The most important of these deposits are associated with the Derrylin Complex. This is a massive belt of sediment banked against the eastern slopes of Slieve Rushen, extending from south of the international border as far north as Derrylin. Dissected ridges cross the low-lying terrain between Slieve Rushen and Upper Lough Erne. The sediments are morainic in origin and record ice activity centred in the Irish Midlands late in the last deglacial cycle. The abrupt eastern slopes of the bank indicate that it was probably in contact with an ice front. Moraine ridges to the northwest of Slieve Rushen indicate that landforms developed during an ice readvance that reached at least as far north as Knockninny. Marked benches in the deposits record a lowering of the ice surface over time. Immediately to the south of Derrylin, the complex consists of a series of clearly defined, low relief, ridge segments and associated kettle holes. Between Derrylin and Upper Lough Erne a prominent ridge, up to 600m wide, marks a halt in the southward retreat of the ice mass. The ridge is defined by broad meltwater channels currently occupied by minor streams. Channels have also been cut across the ridge. Drumlins are found both to the east and the north of the Derrylin deposits and this relationship indicates that this ice-front oscillation occurred later than the drumlin-forming ice flow event. The deposits contain many sand and gravel quarries, the majority of which are now abandoned and grassed.

Key Elements

Deglacial Complexes

DERRYLIN MORAINES complex (6.1Km2)

The Derrylin sediments form the most extensive and thickest glaciofluvial complex in southwest Northern Ireland. The glacial geology of Fermanagh has not been studied in any depth, and it is important that pristine landforms in this area should be preserved.