Omagh Farmland 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 Western River Basins. This region consists essentially of the connected river systems that drain the Carboniferous and Old Red Sandstone plateau of County Tyrone, as well as the foothills of the Sperrin Mountains to the east and Donegal to the west. The region extends from the Omagh Basin in the south, northwards along the lower Foyle valley. The Omagh Basin has particular significance as an ice centre during the Late Midlandian and is now largely covered by a complex mixture of glaciofluvial sands and gravels and drumlins overlying Rogen moraines. When the headwaters of these river systems rise together they have in the past been responsible for serious flooding at the bottleneck of Strabane. Although this has been mitigated by extensive drainage control works in and around the town.

The Omagh Farmland landscape is a drumlin lowland landscape which extends from Omagh to Fintona and includes the river valley landscapes of the Camowen River to the north and the branching river systems of Drumragh River, Ballynahatt Water and Quiggery Water, which wind amongst the drumlins, to the south. The regional town of Omagh is sited at the confluence of the Camowen, Strule and Drumragh Rivers, within a natural bowl-shaped valley that is enclosed to the north by the summit of Mullaghcarn. The landform in this area is relatively broad, with the rivers flowing close to the foot of the Sperrins. To the south of Omagh, the drumlins become the dominant influence on land form, with the sandstone ridges to the east of Fintona and the uplands of West Tyrone forming a distant backdrop. The drumlins are packed quite densely to form a deeply undulating lowland landscape with a distinctive character. Narrow river valleys tend to follow and widen slight gaps between the drumlins and minor tributaries wind around the small hills. There are often patches of marsh in low-lying areas between the drumlins. The drumlins are predominantly pasture but have a diverse, small-scale landscape pattern. In the east of the LCA the landscape is influenced by elements of the Ballygawley deltas complex that is contained within the northeast-southwest upland axis of the Fintona Hills. The landscape can therefore be summarised as one of densely packed with drumlins that create deeply undulating terrain with rounded slopes and numerous small, winding streams, with peaty marsh on some floodplains and occasional small, rounded loughs.

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 Devonian "Old Red Sandstone" rocks of the Fintona Block succession with Carboniferous rocks of the Fermanagh succession to the north. Ordovician igneous rocks and Dalradian metamorphic rocks appear at the LCA margins as fault-bounded successions. Tertiary dolerite dykes extend through the area.

All the above successions were deformed in the Variscan (end Carboniferous) phase of tectonics. The NE-SW Omagh Thrust and Killadeas Fault occur in the north and south of LCA22.

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 (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.

This is a drumlin dominated landscape formed on till that derived from the locally centred Late Midlandian ice mass. 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 and cannot be used determine whether the till is Midlandian or pre-Midlandian in age.

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 identified in the central and southern Omagh Basin, 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. This LCA contains elements of two such deglacial complexes that are important scientifically and for their sand and gravel resources.

The Ballygawley Deltas Complex in the west of the LCA is contained within the northeast-southwest upland axis of the Fintona Hills and extends from Ballygawley in the south to Carrickmore and Sixmilecross in the north. The delta complex is characterised by a well-defined assemblage of sand and gravel delta and outwash surfaces, extending from the Fintona Hills axis northwards, and sand and gravel ridges oriented generally NE-SW around Gortfin. It terminates at the 130 m contour that demarcates the northern sand and gravel limit. Small areas of this complex overlap into LCAs 16, 17, 23 and 45. Most of the complex is in LCA 44.

The Murrins Complex is principally represented by the Coolesker esker in the east of the LCA. The Murrins complex as a whole is located in the lowland area between Carrickmore to the southeast and the upland massif of Mullaghcarn and adjacent hills to the northwest. Associated esker ridges up to 8 km long occur to the south at Beragh, Coolesker, Seefin and Cloghfin. The complex is characterised by proglacial outwash, frontal moraines, small retreat moraines, local high-level deltas and discontinuous feeder eskers and local kettling and meltwater erosion. Most of the complex lies to the north and east in LCAs 43, 23, 24, 25 and 26.

Key Elements

Deglacial Complexes

BALLYGAWLEY DELTA COMPLEX

This landform assemblage is of high importance because of it shows that glaciolacustrine sedimentation took place within the uplands as ice margins withdrew north and south from the Fintona Hills axis. Sediment exposures generally show well-sorted and interbedded sand and gravel facies typical of a glaciolacustrine depositional environment. Correlatable delta and outwash surfaces identify a relative chronology of lake drainage, regulated by ice marginal retreat and the uncovering of points of lake outflow. Sand and gravel ridges around Tiroony indicate that retreat of the northern ice margin was characterised by both active and inactive phases.

Sixmilecross - Tiroony ice marginal complex (BALLYGAWLEY DELTA COMPLEX)

This area is a part of the Ballygawley Delta Complex and runs along the boundary between LCAs 44, 22 and 23. It is of importance in understanding the recent glacial history of Northern Ireland. Landforms consist of ice-marginal sand and gravel ridges that are increasingly hummocky towards the west. Sand and gravel landforms in the Glashagh Burn valley are generally pristine and this intactness is an important landscape attribute. Topographic diversity is the result of the juxtaposition of outwash surfaces, aligned ridges, meltwater channels and bedrock uplands. The morainic system around Sixmilecross is a major landscape feature, and is crossed by the Marshall Country Trail.

Other sites/units identified in the Earth Science Conservation Review

353 Garvaghullion Bog

Peat stratigraphy. Detailed pollen and tephra record. Prehistoric wooden trackway.

AONB

A very small area of the Sperrin AONB (1968) occurs in the north of the LCA.