The Sillees 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 is a lowland area that 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.

In west Fermanagh, the Sillees Valley is a distinct and well-defined lowland area. It is separated from Lough Erne by a ridge of limestone that rises to 200m at Cullen Hill and is bound to the west by the dramatic cliffs of the Knockmore Scarpland. The lowlands are choked with steep-sided drumlins that rise higher and are packed tighter towards the north-west. The Sillees River winds around between the hills through Carran and Ross Loughs to Upper Lough Erne. The drainage pattern is intricate, and the striking pattern of the drumlin hills creates a strong sense of enclosure that is emphasised by the small fields, tall hedges and abundant trees. Gaps between drumlins reveal views over flat wet areas to hills beyond. Key elements in the landscape include: the wide valley of the Sillees River filled with steep sided drumlins.

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 with the exception of the Tertiary dolerite dyke. The Carboniferous formations occur as a roughly northwest - southeast, arcuate striking, discontinuous outcrops, disposed as a linear syncline that is disjointed by east-west faults. The Bundoran Shale is highly fossiliferous (crinoids, brachiopods, bryozoa, corals) and underlies the eastern side of LCA7, being exposed in the Claragh River (ESCR Site 184). Mullaghmore Sandstone Formation and overlying Benbulben Shale are both exposed near Derrygonnelly at ESCR Site 185.

All the above strata were deformed during the end-Carboniferous phase of Variscan tectonic activity. The roughly east-west trending Belcoo Fault occurs in the south of LCA7, where it juxtaposes Benbulben Shale, Mullaghmore Sandstone and Bundoran Shale.

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 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 Sillees Valley is in many respects a classic drumlin landscape. The drumlins are developed in a Late Midlandian till and in the southern part of the LCA they show that ice flow was initially from the east. Some of this flow was then diverted on encountering the Belmore and Ballintempo uplands to flow northwestwards along the present-day Sillees valley. 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 the Sillees Valley, the inter-drumlin hollows have been largely infilled by lacustrine alluvium of sand silt and clay, resulting from the post-glacial flooding of the area that allowed drainage connections between the hollows - most notably by the Sillees River itself.

In the very south of the LCA, where it borders on the Arney Valley, Legg et al. (1998) have identified a number of glaciofluvial deposits associated with subglacial and proglacial proceses during the final deglaciation of the area. These include sand and gravel eskersthat are superimposed upon the drumlin landscape.

Key Elements

Sites/units identified in the Earth Science Conservation Review

184 Claragh

Carboniferous. Exposure of Bundoran Shale Formation.

185 Derrygonnelly

Carboniferous. Exposure of conformable contact between Mullaghmore Sandstone Formation and the succeeding Benbulben Shale Formation. Some fossils.