West Lough Neagh Shores 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 Central Lowlands. This region owes its large-scale morphology to the early Tertiary subsidence of the Lough Neagh basin into the magma chamber from which the basalts that underlie much of the landscape originated. This has produced a largely centripetal drainage system from the rim of the basin into Lough Neagh that ultimately drains northwards via the Lower Bann. To the south of the Lough Neagh basin, the lowlands extend southwestwards along a Caledonian structural trend into the Monaghan-Clones depression. In the east of the region the lowlands extend northeastwards along the fault-guided Lagan Valley. There are no strong topographical barriers in the region and boundaries between LCAs tend to be subtle. The low gradients of the rivers, especially on the clay lowlands immediately around Lough Neagh, create inherent drainage problems and frequently it is only the slopes of the many drumlins that provide permanently dry sites. The Lough Neagh Basin was a major ice accumulation centre during the Late Midlandian and much of the lowland areas to the north and south of the Lough are dominated by extensive drumlin swarms.

The landscape of the West Lough Neagh Shores is found to the east of Cookstown, on the extensive floodplain of the Ballinderry River and its tributaries. The land is relatively low-lying, with a transition from shallow drumlins on the edges of the floodplains, to extensive flat farmland on the fringes of Lough Neagh. Stretches of the rivers are enclosed by embankments and are often hidden from view by extensive woodlands on wet, low-lying land. This LCA is therefore characterised by relatively flat, expansive landscape on the lower reaches of large rivers and fringes of Lough Neagh in which shallow drumlins form 'islands' surrounded by flat, open pastures.

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 extends across the basalt escarpment to the west of Lough Neagh. Here, Cretaceous greensands and limestones or Tertiary basalts rest unconformably on a range of older Mesozoic rock units. The LCA comprises 45% Lough Neagh Group; 45% Lower Basalt Formation, the remainder being the older, underlying units.

The geology appears more complex than it is: being dominated by basalts. The limestones are quarried extensively for lime and aggregate, including the succession of this LCA, the limeworks at Carmean.

Tertiary-aged basalts comprise a crudely-bedded succession of lava flows, columnar jointed lava flows, ash-falls and red-weathered horizons (or boles). The basalts were erupted 55 million years ago as the North Atlantic opened. They are extensively quarried for construction materials, especially roadstone. The palaeoweathered succession of Interbasaltic beds crop out as a small strip on the eastern margin of LCA48.

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 (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.

The drift geology map for this LCA shows that most of it is dominated by till deposits resulting from the drumlin readvance. Away from the lake shore, the landscape of the western half of the LCA is shown in Edwards (1980) to be dominated by large numbers of E-W orientated drumlins that form part of the Western Lough Neagh Drumlin Field. 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 map indicates the presence of small, isolated areas of glaciofluvial sands and gravels associated with the deglaciation of the region and the wasting of the Lough Neagh ice sheet. The map also highlights the alluvial deposits associated with the present-day drainage system and the lacustrine clays and silts of the current Lough shore. The narrow band of lacustrine alluvium around the Lough shore has most probably been exposed by arterial drainage works, such as those completed in 1942, designed to lower Lough Neagh and to improve drainage conditions within the surrounding Basin.

Key Elements

ASSI

030 LOUGH NEAGH

Largest lake in the British Isles, supporting beds of submerged aquatic vegetation with marginal swampy woodland and wet grassland.