Upper Lough Erne 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 Lough Erne Lowland. The depression containing the two lakes cuts across the dominant geological strike of the Old Red Sandstone and Carboniferous Strata that for the most part underlie them. The drift-covered lowland extends considerably beyond the lake shores and the importance of the two basins as ice flow axes during the Midlandian is demonstrated by the drumlins and poorly-drained inter-drumlin hollows that completely dominate the landscape.

Upper Lough Erne is a small-scale intricate landscape in the south of Fermanagh. The landscape is dominated by water as the channel of the River Erne splits and joins, widens and narrows around islands of varying shapes and sizes. Low lying interconnected drumlins and flooded hollows linked by streams and the River Erne stretch across the Lough from west to east providing linkages between the shorelines. Views vary from enclosed inlets to wider reaches of open water that are scattered with islands. The shores are thickly wooded and the surrounding drumlins are divided by a dense patchwork of fields and hedges. Between the drumlins are many small loughs, each fringed with reed beds, carr woodland and the occasional crannog. The only prominent landmark in this vast and intricate network of wetlands is the hill of Knockninny that rises from the drumlin plain and commands excellent views of the Lough. There is a limestone quarry on its western side.

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 of the Fermanagh - Tyrone succession, with the exception of the Tertiary dolerite dyke. The northwest of the area is covered by Upper Lough Erne. The Carboniferous formations occur as a roughly northwest - southeast, arcuate striking, discontinuous outcrops. The Benbulben Shale - Dartry Limestone contact is seen in the Lisnaskea road cutting (ESCR Site 203). The Carboniferous formations were folded and faulted during the end-Variscan (end Carboniferous, about 290 million years ago) phase of tectonic earth movements.

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.

Essentially, this LCA comprises a drumlin dominated lowland underlain by Late Midlandian till, that has been partially drowned by the post-glacial creation of Upper Lough Erne. This has had the effect of producing an intricate network numerous drumlin islands within the Lough. Orientation of the drumlins suggests that ice flow was from the south, although precise orientation depends on local topographic controls (Davies and Stephens 1978). 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.

The Drift geology of the LCA indicates the importance of alluvial deposition, along the sinuous channels that skirt the drumlins.

Key Elements

Sites/units identified in the Earth Science Conservation Review

203 Lisnaskea

Carboniferous. Exposure of contact between Benbulben Shale formation and succeeding Dartry Limestone Formation. Some fossils.