Slievemore 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 Uplands of Tyrone and Fermanagh. This area is defined in the north by the fault-guided scarp that forms the southern edge of the Sperrin Mountains. Below this are plateau lands that decrease in height and complexity to the south, before rising again to the lower slopes of Slieve Beagh. Below ca 350m the landscape is dominated by thick drift deposits, including prominent drumlin fields, dead ice features and glaciofluvial deposits - often capped by blanket peats. Some hills rise above the general level of the plateau, most notably the basalt-capped outlier of Slieve Gallion. The southwestwards trending Clogher Valley effectively divides the southern section of the upland into two blocks, one lying between Tempo and Pomeroy and the other centred on Slieve Beagh.

Slievemore is the summit of the sandstone ridges that dominate the landscape of south east Tyrone. They extend from Pomeroy, on the Creggan Plateau, to Garvaghy and the escarpment at Errigal. The ridges are underlain by a variety of different sandstones, including Lower Old Red Sandstone and Conglomerates. The summits of Slievemore (313m) and Cappagh Mountain (280m) have a broad, rounded landform, while the lower slopes have a more complex, hummocky land form, a result of glacial deposition. The land form near Pomeroy has a slightly different character. Here a high plateau, which is underlain by resistant igneous rocks, is deeply dissected by valleys such as that of the Upper Claggan River. The landscape has a diverse pattern, relating closely to the varied topography. The broad ridgetops are a large-scale mosaic of moorland and conifer plantations, with smaller pastures on the marginal slopes. Peat cutting has had a significant influence in many parts of the moorland. This is a marginal upland landscape. Quarries and derelict sand and gravel workings are relatively common elements, and upstanding transmission masts on the open moorland are very prominent in views from the surrounding lowlands. The landscape can therefore be summarised as one of broad, rounded sandstone ridges with a steep escarpment to the southwest, and glaciiofluvial deposits within deep glens and on lower slopes that produce an undulating, irregular landform, broken by rocky outcrops. Key elements in the landscape include the wide range of deglacial deposits of the Ballygawley and Pomeroy complexes. The Ballygawley delta complex is contained within the northeast-southwest upland axis of the Fintona Hills that are approximately asymmetric in cross profile with glacial sediments lying within upland bedrock passes up to 190 m elevation. North of Ballygawley, the southeastern flank of the Fintona Hills is characterised by steep bedrock slopes dipping into the Clogher Valley. The northwestern flank of the Fintona Hills is characterised by fluvially dissected sand and gravel spreads. The Pomeroy complex is classed as aesthetically poor due to the large-scale commercial sand and gravel extraction operations, visible from main roads throughout the area. This is despite the topographic diversity added to the area by deep, wide, steep-sided meltwater channels, prominent valley-side ridges and remnants of flat-topped, dissected valley-fills.

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)

Tertiary - dolerite dykes & intrusions, about 60 million years old
Carboniferous - about 350 million years old
Maydown
Clogher Valley
Ballyness
Devonian - about 400 million years old
Gortinfinbar, Shanmullagh, Shanmaghery
Silurian - Little River Group - about 440 million years old
Late Caledonian - Tremoge Glen Granite - about 400 million years old
Early Caledonian - Tyrone Plutonic Complex - about 450 million years old
Ordovician - Desertcreat Group - about 450 milion years old

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 confirms the drift free nature of the upland areas within the LCA and their extensive blanket of peat. The surrounding lowlands carry a cover of Late Midlandian till, but the topographic diversity is largely a product of deglacial processes. These have resulted in the presence within the LCA of elements of two major 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 northeast-southwest around Gortfin. It terminates at the 130 m contour which demarcates the northern sand and gravel limit. Small areas of this complex overlap into LCAs 16, 17, 23 and 45 with a more substantial element in 22.

The Pomeroy Deglacial Complex in the north centre of the LCA is a 118 km2 area in the Fintona Hills containing units of glaciolacustrine and glaciofluvial sediments in the valleys and along the hillsides of glacially moulded bedrock highs. The area occurs along the watershed between the major depressions of the Omagh basin and Lough Neagh lowlands. The deposits and landforms record regional scale deglaciation patterns and provide evidence for the formation of localised and regional-scale water bodies along the deglaciated southern margins of the Sperrin mountains. This deglacial phase occurred during the eastward and westward retreat of ice margins generally towards the depressions of the Lough Neagh and Omagh Basins. The complex also extends northwards into LCA 43.

Key Elements

ASSIs

The LCA includes a small element of the BARDAHESSIAGH ASSI

097 LITTLE RIVER (ca 50%)

An important historical palaeontological locality. The type locality for over 60 fossil species including 2 important genotype species. It is one of only two localities in Northern Ireland exhibiting a shelly fauna in a Silurian graptolitic setting.

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. Northeast-southwest oriented sand and gravel ridges around Tiroony indicate that retreat of the northern ice margin was characterised by both active and inactive phases.

BALLYMACKILROY delta and outwash (Ballygawley DELTA complex)

The area around Ballymackilroy in the extreme south of the LCA overlaps with LCAs 16, 17 and 45 and is of importance in understanding the recent glacial history of Northern Ireland. It consists of a well-defined assemblage of sand and gravel delta and outwash surfaces deposited in a pro-glacial lake. Landforms around Ballymackilroy record sand and gravel deposition following the breakup of ice over the Fintona Hills and subsequent ice withdrawal towards the south, into the Clogher valley.Sand and gravel landforms around Ballymackilroy are generally pristine and this intactness is an important landscape attribute. The outwash spreads are a major component of the landscape as seen from the main -Omagh road.

Sixmilecross - Tiroony ice marginal complex (Ballygawley DELTA complex)

This area is a part of the Ballygawley Delta Complex and runs along the boundary between LCA 44 and LCAs 22 and 23 to the north. The area between Sixmilecross and Tiroony 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.

POMEROY DEGLACIAL COMPLEX

The complex is rated as having a good scientific importance because it provides a records of regional ice pressure from the Omagh basin area during the last glacial cycle and of a proglacially ponded water body at 245 m O.D. during the deglaciation of the area, It also demonstrates ice-marginal retreat westwards into the Omagh basin and eastwards into the Lough Neagh lowlands towards the end of the last glacial cycle. Deglacial landform associations include feeder channels and a proglacial fan, meltwater channel incision events and infills, and an excellent example of a major morainic tract at Pomeroy.

EDENFORE glaciofluvial COMPLEX (POMEROY COMPLEX)

Landforms in the Edenfore area are a component of the Pomeroy Complex, a series of glacio-fluvial landforms along the watershed between the Omagh and Lough Neagh Basins. Deposits at Edenfore consist of a flat-topped, dissected, steep-sided delta and deeply-dissected outwash. Dissected terrain at lower levels add diversity to the landscape. The deltaic deposits are a prominent feature overlooking the Ballygawley-Sixmilecross road. Aggregate extraction is confined to conspicuous workings in the most northerly part of the deltaic deposits.

ANNAGH ESKER (BALLYGAWLEY DELTA COMPLEX)

A well-marked, branching, sinuous ridge is aligned northeast to southwest immediately to the north of Annagh in the east of the LCA. The ridge is an esker that records sediment transport to ice front positions marked by moraine ridges on lower ground to the north. The esker and its spatial relationship with adjacent morainic landforms are evidence for retreat of the eastern Omagh Basin ice margin and assist in understanding the complexity of deglacial processes.

KILLEY BRIDGE AND TIRNASKEA ASI

Other sites/units identified in the Earth Science Conservation Review

Ordovician

Desertcreat Group

Fossiliferous sandstones, limestones and mudstones in an east-west trending, 1.5km-wide outcrop in the northeast corner of LCA44. Includes Bardhessiagh ASSI (096): Caradoc age fossil location.

Silurian

Little River Group

Mudstones and siltstones with graptolite fossils. Forms two east-west trending tracts in the northeast of LCA44, adjacent to the Desertcreat Group. Includes Lime Hill Farm ASSI (098), the continuation of the type location at Little River.

Devonian (Upper Palaeozoic)

Shanmaghery Formation (Upper Devonian) - Sandstones with andesites, exposed in the far western edge of LCA43 in faulted contact with Gortinfinbar Formation. The Barrack Hill Andesites for part of this formation and are more widespread in the southeastern corner of LCA44, where they underlie to Gortinfinbar Formation. These conglomerates cover the central 80% of LCA44. The Gortinfinbar is exposed at Lurgylea, ESCR Site 58, where dated lavas are exposed.

All the above successions were deformed in the Variscan (end Carboniferous) phase of tectonics. The NE-SW Tempo-Sixmilecross Fault crosses the northwestern edge of LCA44.