Beaghmore Moors and Marsh 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.
The Beaghmore Moors and Marsh LCA is a relatively elevated, rolling plateau of wide shallow valleys and broad, rounded ridges to the south and east of the Sperrin Mountains. Extensive glacial deposits form irregular ridges and mounds throughout the area. Slopes typically have shallow, smooth profiles, although some quarried outcrops have an irregular skyline. This is an expansive, relatively homogeneous landscape, fragmented in some areas by small conifer shelterbelts protecting farmsteads. Extensive conifer plantations on the shallow valley slopes often mask the land form. There are some deciduous woodlands in gullies on valley sides and the incidence of woodland increases towards the slopes of the Sperrins to the north west. The area is pitted with sand and gravel quarries. The landscape can therefore be summarised as one of shallow low ridges of glacial moraine separated by extensive peaty marsh and numerous winding small rivers. A key element in the landscape is the Murrins degacial complex in the southwest of the LCA. This is a visually intact moraine-outwash assemblage that provides landscape contrasts through the juxtapostion of sand and gravel and bedrock and differences in ridge orientation and topography type. Moraines and feeder eskers also contrast with intervening alluvial flats, meltwater channels and kettleholes. The most significant pressure for change is from the large, modern sand and gravel quarries, most of which are close to the A505. The plant, machinery and vast spoil heaps associated with these quarries have a wide visual influence in this relatively expansive, rolling landscape.
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)
Carboniferous - about 350 million years old |
|---|
Omagh |
Ordovician - about 450 million years old |
Tyrone Volcanic Group |
Tyrone Plutonic Complex |
Un-named Caledonian intrusives |
Laght Hill Tonalite |
Dalradian (Neoproterozoic) Mullaghcairn Formation - about 600 million years old |
Moinian (Tyrone) Corvanaghan Formation - about 1000 million years old |
LCA25 is dominated by igneous rocks of the Tyrone Volcanic Group and Tyrone Plutonic Complex. These Ordovician rock units possess a NE-SW fabric, in parallel with the Omagh Thrust which parallels the northwestern edge of the LCA and brings Dalradian Mullaghcairn schists (metamorphic rocks) into contact.
The Ordovician Tyrone Volcanic Group comprises an early Ordovician series of tuffs, lava flows, pillow lavas. Volcanic rocks of this succession are exposed at Beaghbeg (ESCR Site 430). Tuffs and breccias associated with the Copney Pillow Lava succession are seen at Creggan (a series of crags - ESCR Site 428). The Copney Pillow Lavas themselves are seen at the ESCR Site 427, Mweela More. Various Tyrone Volcanic Group successions are seen at ESCR Site 429 Copney Hill. Tyrone Plutonic Group gabbros and dolerites are exposed at ESCR Site 425 (Black Rock).
The Laght Hill Tonalite is exposed at ESCR Site 434 - a crag named Cashel Rock, where deformed rhyolites are also observed.
Two tectonic phases have affected the area: the Caledonian (Ordovician - Silurian) and Variscan (end Carboniferous). Consequently NE-SW faults, folds, intrusions and minor fabrics posses this orientation. The Omagh Thrust runs along the northern edge of LCA25.
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.
The drift geology map shows that much of the west of the LCA is underlain by till derived from a Late Midlandian ice mass. This was centred to the southeast in the Omagh Basin and moved northeastwards across the area to form the central Tyrone drumlin field. It is in the east of the LCA, however that the features of greatest geomorphological and geological interest are to be found. This area represents a complex boundary between Lough Neagh centred ice that approached the area from the southeast and Tyrone ice coming from the southwest. This interaction reveals itself in the local complexity of the landscape and its associated drainage. This complexity derives largely from an often-chaotic arrangement of deglacial deposits formed by the wastage of the ice sheets and retreat of their margins towards their source areas. Within the LCA there are significant elements of three major deglacial complexes that are important scientifically and for their sand and gravel resources.
The Murrins Complex occupies 6.4km2 in the west 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. The northern part of the complex terminates at the foothills of the Sperrin Mountains near Greencastle. Associated esker ridges up to 8 km long occur to the south at Beragh, Coolesker, Seefin and Cloghfin. Sediments are mainly superimposed upon bedrock to the west and north, and glacial till to the east and south. 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. The complex also lies to the south and west in LCAs 43 and 26, with outliers to the southwest in LCAs 22 and 23.
The Ballinderry River Valley Complex occupies 2.4km2 in the south of the LCA and are part of an area of predominantly lowland, extensively quarried glaciolacustrine sand and gravel to the southeast of the main Sperrin mountains range. The thickest deposits of stratified drift occur along the flanks of the Ballinderry river valley and on the southern highland margin of this tectonic bedrock depression. The complex is scientifically important because it demonstrates the localised southward and eastward retreat of ice and more general regional deglaciation patterns. Intact, intricate, topographic associations have been preserved including excellent examples of subglacial sediment supply and proglacial sedimentation at Lough Doo and Cam Lough. Deglacial landforms including esker complexes, Gilbert-type deltas, ice-contact slopes, outwash terraces, glaciotectonically influenced cross-valley ridges are preserved and the complex provides examples of the effects on ice-mass decay patterns of bedrock topography. Most of the complex lies in LCA 43 to the south.
The Lough Fea Deglacial Complex occupies a substantial area in the northeast of the LCA, but most of the Complex lies to the north within LCAs 40 and 41. An intermittently stagnant ice front has left a classic series of deglacial landforms around the Lough Fea area. Initial ice halts are marked by moraine ridges south of Lough Fea, at Lissan and Grouse Lodge, where there is also evidence of sediment deposition within a waterbody. Retreat continued to positions at Mill Lough and north of Davagh Eskers, again marked by moraine ridges. During this latter phase, subglacial channels fed sediment and meltwater under the ice mass and over the Teal Lough and Brackagh outwash plains, which were ice-free by this time. The line of this channel is marked by sections of the Davagh esker. It is extremely unusual to find all features associated with this ice disintegration so well preserved in such close proximity.
The poor drainage associated with the hummocky terrain of the LCA is reflected in the large areas of peat that are shown on the Drift Geology map.
Key Elements
ASSI
011 TEAL LOUGH & SLAGHTFREEDEN BOGS
Three sites of undisturbed upland raised bog within blanket bog. Fine hummock and pool complex at Teal Lough. Bog pools contain uncommon plant species. High Sphagnum cover.
Deglacial Complexes
The Murrins complex
The Murrins complex records a sequence of sedimentary events associated with southwestern ice retreat from the southern Sperrin Mountains into the central part of the Omagh Basin and can be classified as being of unique importance on a Northern Ireland scale.
CAM LOUGH/SULTAN delta
This area of the Ballinderry sand and gravel landscape occupies the south-centre of LCA 25 and overlaps with LCA 43 to the south. The Cam Lough area is of importance in understanding the recent glacial history of Northern Ireland and contains excellent examples of a range of deglacial landforms including eskers, flat-topped deltas, ice-contact slopes and kettle holes. The relationship between the esker and delta series is clearly demonstrated while the ice-contact slope is pristine. The Cam Lough and Sultan complex is among the largest remaining, intact, sand and gravel units in Northern Ireland. The basinal lowland containing Cam Lough is enclosed to the west by the steep slope of a sharp-crested, hummocky cross-valley ridge that extends from the foothills of Cregganconroe to Sultan.
OWENBRACK/CLOGHFIN RIVER VALLEY PROGLACIAL ASSEMBLAGE (MURRINS COMPLEX)
This limited set of deposits straddle LCAs 24 and 25 and consist of a well-defined assemblage of sand and gravel delta and outwash surfaces deposited in a pro-glacial lake. They show that sediment was carried by meltwater derived from an ice front situated on high ground to the south. This meltwater must have been dammed by an ice front on lower ground to the north. The landforms are generally pristine and this intactness is an important landscape attribute. It is of high scientific importance because it provides evidence for the pattern of ice mass fragmentation as the ice thinned.
LOUGH FEA DEGLACIAL COMPLEX
The Lough Fea area is a classic grouping of associated deglacial landforms that can subdivided into discreet assemblges. This LCA includes a number of important sites including the Lough Teal outwash plain with moraine ridges and the Brackagh outwash plain. This LCA comprises primarily the Davagh esker and elements of the Lough Teal Plain.
Other sites/units identified in the Earth Science Conservation Review
430 Beaghbeg
Tyrone Volcanic Group. Quality exposures of a variety of tuffaceous volcanic lithologies.
425 Black Rock
Tyrone Plutonic Group. Outcrops of lithology of gabbro and dolerite. Part of the ancient Iapetus Ocean floor.
434 Cashel Rock (Laght Hill Tonali
Caledonide-Igneous. Tyrone Volcanic Group. Outcrops of tonalite and rhyolite showing lithology and deformational variation.
428 Creggan
Caledonide-Igneous. Tyrone Volcanic Group. Quality outcrops of volcanic tuffs and breccias. Closely associated with Copney Pillow Lava Formation.
427 Mveela More
Caledonide-Igneous. Tyrone Volcanic Group. Outcrops of ice-sculpted basaltic pillow lava. Part of Copney Pillow Lava Formation.
429 Copney Hill
Caledonide-Igneous. Outcrops of various lithologies typical of Tyrone Volcanic Group.
AONB
All of the LCA lies within the Sperrin AONB (1968). This designation is indicative of the scenic quality of the landscape.
