Upper Moyola Valley 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 Upper Moyola Valley is the broad basin of the Moyola River on the eastern fringes of the Sperrin Mountains. It is dominated by the peak of Slieve Gallion to the south. The valley is deeply undulating, with steep, rounded slopes dissected by numerous small branching streams. The slopes become steeper and smaller in scale towards the centre of the valley. The surrounding mountains provide a strong sense of enclosure. The steep slopes towards the valley centre have a relatively small-scale landscape pattern. The landscape becomes larger in scale towards the outer margins of the valley, where many of the fields are enclosed by stone walls. This regular field patchwork extends high up onto the slopes of the surrounding mountains, often with a striking division between the farmland and the moorland above. The landscape can therefore be summarised as one of a broad, undulating limestone valley, dissected by numerous small, branching streams. Key elements in the landscape include a number of glaciofluvial complexes. In the east of the LCA, the Moyola complex is aesthetically important because of the topographic diversity provided by visually prominent ridges, interspersed with meltwater gashes and broad alluvial spreads on the flanks of the ridges and the location of the deposits in a broad fault-bounded depression bordered by the heather covered Sperrin highlands. In the south of the LCA, the Lough Fea area contains an assemblage of associated deglacial landforms that include eskers, end moraines, outwash plain and kettle depressions. It is extremely unusual to find all these features associated with this ice disintegration so well preserved in such close proximity.
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 - Barony, Iniscairn, Altagoan, Desertmartin |
|---|
| Ordovician - Tyrone Volcanic Group |
| Dalradian - various |
Comprises three ages of rock strata: Neoproterozoic ("Dalradian"), just under 1/3 in the west of the LCA; Ordovician (Lower Palaeozoic), a strip in the south of the LCA and Carboniferous (Upper Palaeozoic) clastic and carbonate sediments (including Drumard Water, described in ESCR Site Report 255), about 1/3 of the east of the LCA. These three successions are usually in faulted contact.
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 much of the landscape is dominated by Late Midlandian till, resulting from the expansion of Lough Neagh centred ice. However, the Quaternary features that are of greatest geomorphological and geological significance are located within areas of deglacial sands and gravels. Elements of two major deglacial complexes occur within the LCA that are important scientifically and for their sand and gravel resources.
The Moyola Valley Complex covers an area of in the east of the LCA and consists largely of subglacial, glaciolacustrine, morainic and alluvial sediments on low ground in the Moyola river valley. The rising ground of the Sperrin mountains define the margins of the valley. The Moyola river drains the eastern margins of the Sperrins towards the lower Bann river. Sand and gravel landforms consist of pristine flat-topped ridges, dissected flat-topped ridges, large- and small-scale linear cross-valley ridges and isolated meltwater eroded hummocks. These are surrounded by flat, extensive alluvial spreads. A small area of the complex overlaps with LCA 50 to the east.
The Lough Fea Deglacial Complex occupies an area 3.4km2 in the southwest of the LCA, with most of the Complex in LCA 41 and a smaller area in LCA 25. 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.
Key Elements
Deglacial Complexes
MOYOLA VALLEY DEGLACIAL COMPLEX
The Moyola Valley complex is scientifically important through providing a record of complex proglacial landform deposition during during the last deglacial cycle. The preserved morphology records proglacial waterbodies and landform patterns and palaeoflows record the general eastward retreat of the ice-lobe that occupied the valley during the last deglacial period. It also provides evidence in support of models of a generally south and eastward retreat of ice from the Bann Valley towards the Lough Neagh lowlands and toward the eastern flanks of the Sperrin Mountains.
BLACK HILL Ridge (MOYOLA VALLEY DEGLACIAL COMPLEX)
A marked ridge, together with associated rolling terrain across the Moyola River valley to the east of Draperstown, stretches between the lower slopes of Slieve Gallion and Slievemoyle. Landform patterns record the general eastward retreat of ice into the Lough Neagh lowlands towards the end of the last deglacial cycle. Meltwater dissection has resulted in the development of markedly hummocky, irregular terrain to the south of the main ridge.
BELMOUNT HILL SAND AND GRAVEL SPREAD (MOYOLA VALLEY DEGLACIAL COMPLEX)
An extensive, dissected spread of sand and gravel inferred to result from the deposition of sediments in an ice-marginal lake located at the southeast entrance of the Moyola River valley. Landforms record deglacial processes during ice withdrawal towards the Lough Neagh Basin. The adjacent Black Hill area is an excellent example of the landforms produced at a time when glacier ice was increasingly confined to low altitude valleys and basins. A limited area of this outwash overlaps with LCA 50.
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 parts of the Lough Teal outwash plain with moraine ridges and the Brackagh outwash plain shared with LCA 41. The third major element, the Davagh esker lies mainly in LCA 25.
Other sites/units identified in the Earth Science Conservation Review
255 Drumard Water
Carboniferous stratigraphy of Drumard Member.
258 Drumbally Hill
Exposures of the fossiliferous Desertmartin Limestone Formation are exposed in Drumbally Hill. Some crinoid and brachiopod fossils.
257 Altagoan River
Early Carboniferous stratigraphy and faulting in Altagoan Formation. Uppermost contain several different groups of fossils. Ripple-marked and cross-stratified sandstones and siltstones are seen in Altagoan River.
432 Tullybrick
Ordovician: deformed (Tyrone Volcanic Group) tuffs and phyllites are exposed in Tullybrick valley. Evidence of cataclastic deformation.
288 Moyola River
Lower Carboniferous Palaeontology. Exposure of parts of Mormeal Member of Altagoan Formation. Sediments contain rich and diverse, though patchy, early Carboniferous fossils, particularly fish.
AONB
The western half of the LCA lies within the Sperrin AONB (1968). This designation is indicative of the scenic quality of the landscape.
