Lower 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 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.

Lower Lough Erne is the more dramatic of the Lakeland loughs, with extensive open waters, offshore islands and the bold escarpment of the Magho Cliffs to the south. This character area covers the northern part of the Lough, which stretches for 20 miles from Enniskillen to Rosscor. The Lough lies in a deep glacial trough, the alignment of the shores and islands reflecting the direction of ice movement. On the southern shores ice-scored limestones and ancient quartzite rocks form prominent scarps, the Magho Cliffs, but further north the low streamlined ridges, bays and promontories of Boa Island and Castle Caldwell are less pronounced. Below the Magho Cliffs a narrow strip of farmland and alder woodland borders a boulder strewn shore. Key characteristics of the landscape include: highly valued lakeland scenery of outstanding quality, the dramatic limestone cliffs and escarpment that contain the Lough on the south and islands and Lough shores rich in monuments and ecclesiastical sites, including raths, crannogs, ruined castles and numerous listed buildings.

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 comprised of fossiliferous Carboniferous sedimentary rocks resting in faulted and unconformable contact with Moinian basement. Most of the LCA is covered by Lower Lough Erne. Overall, Mullaghmore Sandstone forms the most extensive rock type (including the Lough Erne subsurface) and the succession dips south. The Carboniferous limestone and shale are exposed in Magherameena (Magheramenagh) Quarry (ASSI 119). The Benbulben, Glencar and Dartry are exposed in the Cliffs of Magho ASSI (ESCR Site 189).

The Moinian forms some of the oldest rocks in Northern Ireland. They are metamorphic schists. Exposed at Gadalough (ESCR Site 310), with serpentinised relics of basic igneous intrusions.

The Carboniferous Keenaghan Shale is best known from the Tyrone - Fintona Block Carboniferous so its presence here is unusual. Rests unconformably upon Lough Derg Group at Keenaghan Lough (ESCR Site 209). All the Carboniferous strata were deformed during the end-Carboniferous phase of Variscan tectonic activity. Major faults and folds have their strike ENE-WSW.

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 >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 epoch. 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.

Essentially, this LCA comprises a drumlin-dominated lowland underlain largely by Late Midlandian till, that has been extensively drowned by the post-glacial creation of Lower Lough Erne. The Lough occupies a former glacial valley that was eroded well below present-day sea level. This has had the effect of producing numerous drumlin islands around the margins of the Lough. Orientation of the drumlins suggests that ice flow was from the east, 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.

The drift geology map of the LCA clearly shows the manner in which the western end of the Lough is closely confined between the limestone cliffs of Magho to the south and the Pettigoe Plateau to the North. Post-glacial adjustment in the landscape is also indicated by the extensive area of landslip mapped below the Cliffs of Magho.

Key Elements
 

ASSIs

119 MAGHERAMENAGH

It is one of only two Waulsortian mound limestones in Northern Ireland . This site is the most northerly occurence of the Waulsortian in Ireland , Great Britain and Western Europe

191 The Cliffs of Magho (ca 50% shared with LCA 4)

The Cliffs of Magho consists of a large, north-facing limestone escarpment overlooking Lower Lough Erne. The area includes the dip slopes and a series of smaller sandstone scarps and ridges behind the main cliffs. The continuous nature of the exposure is exceptional and allows the relationships between the strata to be seen in great detail. The limestones and shales exposed are of Lower carboniferous age and the cliffs are the most northerly occurrence of the Glencar and Daltry formations.

Other sites/units identified in the Earth Science Conservation Review

310 Gadalough

Precambrian. Lough Derg Group. Unusual altered and serpentinized relics of basic igneous intrusions.

209 Keenaghan Lough

Carboniferous. Exposure of plane of unconformity between Keenaghan Shale Formation and Lough Derg Group.