Enniskillen 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.

This landscape unit includes the southern end of Lower Lough Erne, the town of Enniskillen and the winding rivers and wetlands in the northern part of Upper Lough Erne. It is a landscape of open water, wooded islands and richly vegetated shorelines, which is often invisible from the main roads that pass around the Lough. The Lough and riversides are dominated by grassy drumlins, with fields separated by bushy hedgerows and hedgerow trees and numerous wooded off-shore islands. The lough shore landscapes and their associated semi-natural vegetation would be highly sensitive to development. Elsewhere there is scope for change to be accommodated within areas with undulating topography and a robust landscape pattern (intact hedgerows and woodland). Enniskillen is sited on a series of drumlin islands and bridges the River Erne at its inflow to Lower Lough Erne. The LCA as a whole can be summarised as an attractive lakeland with complex shorelines, promontories and wooded islands surrounded by high drumlin farmland. Key elements in the landscape include Fardrum Turloughs on mid-western margin of LCA.

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 fossiliferous Carboniferous rocks of the Fermanagh succession to the south and of the Kesh - Omagh succession to the north. These Carboniferous strata overlie Devonian and Silurian, exposed in small outcrops. Tertiary dolerite dykes extend through the area. Silurian - Lisbellaw Formation greywackes and shales exposed in the southeastern corner of the LCA in faulted contact with Carboniferous, Devonian - Shanmullagh Formation (probably Upper Devonian) red sandstones, mudstones and siltstones with subordinate andesitic lavas typical of the Old Red Sandstone facies underlie the northeastern corner of LCA13, in faulted contact with Carboniferous to the north and unconformably overlain by Carboniferous to the west. A Ballyshannon Limestone Waulsortian mudmound reef complex in the southern part of LCA13 is in Bellanaleck ASSI 120 and is one of only two designated Waulsortian reefs in N.Ireland: late Tournaisian trilobites are recorded. Exposed at ESCR sites 180 (Blaney Quarry); 181 (Carrickreagh) and 182 (Inisway) where abundant fossils are recorded.

NW-SE oriented Tertiary dolerite dykes extend through LCA13: one exposure is recorded (ESCR Site 83) at Doraville Ridge.

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 iceflows 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 by Late Midlandian till, that has been partially drowned by the post-glacial creation of Lower Lough Erne. This has had the effect of producing 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 clearly indicates how the glaciated lowland is closely constrained to the southwest by the limestone uplands, which map as drift-free bedrock. The map also indicates the importance of alluvial deposition, especially south of Enniskillen.

Key Elements

ASSI/ASIs

138 FARDRUM & ROOSKY TURLOUGHS

Fardrum and Roosky are the only positively identified turloughs in Northern Ireland and the most northerly in Ireland. These limestone lakes, which dry out in summer, support rare flora in the inundation zone, while the permanently wet basins within the turloughs support vegetation typical of lakes and lake shores. Although relatively common in County Clare, there are few examples elsewhere in Britain and Ireland. They are in a good state of preservation and are of significant geomorphological and geological interest and quality in a Northern Irish context.

120 BELLANALECK

One of the most important geological localities in the Carboniferous outcrop of Northern Ireland being one of only two occurences of Waulsortian mound limestones . A diverse macrofauna includes, for the first time in Northern Ireland, a suite of trilobites that are of late Tournaisian age, some 340 million years old.

(ESCR site 83) Doraville Ridge ASI

Tertiary feeder dyke of olivine tholeiite, noted for its mineralogical interest.

Other sites/units identified in the Earth Science Conservation Review

180 Blaney Quarry

Carboniferous. Fossiliferous exposure of Ballyshannon Limestone Formation.

181 Carrickreagh

Carboniferous. Exposure of lower and middle sections of Ballyshannon Limestone Formation.

182 Inisway

Carboniferous. Exposure of upper fossiliferous layers of Ballyshannon Limestone Formation.