Newtonbutler and Rosslea Lowlands  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 Newtownbutler and Rosslea Lowlands lie between the Finn and Lacky Rivers and the Carnock and Mullaghfad Hills. Lines of drumlins form undulating ridges that extend across the area from west to east, influencing the pattern of rivers, lines of communication and settlement. The drumlins form low rounded hills or ridges rising above wet intervening hollows that often have small loughs. Open views in all directions are bounded by low drumlin hills but the River Finn to the south, has an extensive floodplain. North of the B36 the drumlins increase in height and are more tightly packed together with steep intervening valleys. The relatively wooded character of the thorn hedges gives the landscape a well-vegetated appearance. At the foot of the Carnrock Hills, there is a more pronounced valley and a chain of small loughs, with fringing reed beds, carr woodlands and wet meadows. Uninterrupted pastures, tall overgrown hedges, small farms and winding roads give this landscape a rural character. The scale of the farm and landscape pattern varies to reflect drainage and management. The River Finn is claimed to be the finest lowland river in Northern Ireland and the Finn and the Lacky Rivers are amongst the select few rivers in the country that have unaltered courses.

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 dykes.

The Carboniferous formations occur as a roughly northeast - southwest, arcuate striking, discontinuous outcrops. The Ballyshannon - Bundoran succession (including Newtownbulter Limestone Member and Mullynagowan Sandstone Member) are exposed in the Newtownbulter Stream (ESCR Site 201). Benbulben Shale exposed in the deep ditches of Kilmore and Drumbullog (ESCR Site 202).

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. 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.

This LCA is a classic drumlin landscape, dominated by Late Midlandian till that was deposited by ice that moved across the region from a centre in the northeast. 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.

In recent years, researchers have also identified in this area a great many subglacial diamict (till) ridges that lie transverse to the southwestwards Late Midlandian ice flow (e.g. Knight and McCabe 1997). These ridges can be anything from 0.5 - 2.5km in length and 100 - 450m wide and have been interpreted as rogen moraines. Some of ridges were streamlined and overprinted by subsequent drumlin development, while others remained unaffected. In some cases eskers were draped across the ridges during final deglaciation. This combination of subglacial bedforms is used to suggest that during the last deglacial phase ice masses were highly mobile and that flow was episodic due to variations in the subglacial thermal regime. One consequence of this was a high sediment flux to the ice margins that in turn generated significant glaciofluvial complexes.

Key Elements

Sites/units identified in the Earth Science Conservation Review

201 Newtownbutler Stream

Carboniferous. Exposure of contact between Newtownbutler Limestone and Mullynagowan Sandstone Members with Ballyshannon Limestone and Bundoran Shale Formations. Fossils of corals, brachiopods and foraminifera.

202 Kilmore and Drumbullog

Carboniferous. Exposures of fossiliferous lithologies of Benbulben Shale Formation.