Dungannon Drumlins and Hills 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 Dungannon Drumlins and Hills is a drumlin landscape, but with an unusually large scale land form. The drumlins form steep, rounded hills, some of which are linked together to form groups of hills with distinctive, rounded summits. They show no particular alignment. In more elevated areas, the low-lying land between the drumlins is marshy, often with rounded loughs that help to accentuate the height of the drumlins. Aghnahoe Hill, Ivy Hill and The Burnt Hill reach an elevation above the surrounding drumlins and offer welcome long views. To the south and west, towards Augher and Caledon, there are areas of low-lying land. Here the high drumlins are separated by a gently rolling lowland plain. In places there are areas of marsh, but most of this area has been drained to produce good agricultural land and arable fields are interspersed with pastures. A feature of the north of the LCA are elements of the Moneymore deglacial complex that consists of discontinuous sand and gravel mounds and spreads in a north/south oriented belt between Moneymore, Coalisland and Dungannon. The landforms are dissected and consist predominately of ice-marginal and extra-marginal glaciolacustrine sequences superimposed upon the western Lough Neagh drumlin field.
The Moneymore complex consists of discontinuous sand and gravel mounds and spreads in a north/south oriented belt in the Lough Neagh lowlands to the east of the Sperrin highlands between Moneymore, Coalisland and Dungannon, Co. Tyrone. The landforms are dissected and consist predominately of ice-marginal and extra-marginal glaciolacustrine sequences superimposed upon the western Lough Neagh drumlin field. The stratified deposits formed in association with localised and variable palaeolake levels during the final deglacial stages of the region. Sediment supply was largely from a small, lowland residual ice-mass within the Lough Neagh basin and meltwater draining the eastern margin of the Sperrin highlands.
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 - Lough Neagh Group, about 20 million years old |
|---|
| Tertiary - Lower Basalt, about 55 million years old |
| Tertiary - dolerite dykes and sills, about 60 million years old |
| Cretaceous - greensands and limestones - about 100 million years old |
| Triassic - Mercia Mudstone Group and Sherwood Sandstone Group - from about 240 to 210 million years old |
| Carboniferous - Ballyness, Bundoran, Blackwater, Carrickaness, Clogher Valley, Carboniferous Limestone (undifferentiated), Maydown, Rockdale, Rossmore, Millstone Grit (undifferentiated), Coal Measures (undifferentiated) - from 350 to 290 million years old |
| Devonian - Shanmaghery, Gortinfinbar, Barrack Hill - about 400 million years old |
| Late Caledonian - Slieve Gallion Granite - about 400 million years old |
| Early Caledonian - Tyrone Plutonic Complex - about 450 million years old |
| Moinian - Corvanaghan - about 1000 million years old |
Comprises a large number of rock types of many ages in complex faulted, unconformable and intrusive relationship. The Carboniferous rocks dominate and comprises three successions in faulted contact (see appendix for discussion).
The Fermanagh succession comprises Ballyness Formation (conglomerates and sandstones); Clogher Valley Formation limestones and shales and Bundoran Shale Formation (exposed in Drummond Quarry, ESCR Site 232).
The Dungannon succession comprises Carboniferous Limestone (the basal part of which is at least partly equivalent to the Bundoran Shale), the Rockdale Limestone, Rossmore Mudstone, Millstone Grit and Coal Measures (seen in Coalisland Brick Pit: ESCR Site 247). The Rossmore Mudstone is exposed in Derraghadoan Pit. NE-SW faults occur throughout the Carboniferous and bring into contact with the Dungannon succession parts of the Armagh Carboniferous (Maydown Limestone, Carrickaness, Blackwater Limestone). Fossiliferous (corals and brachiopods) Maydown Limestone is seen at Plaister Quarry (ESCR Site 234). The Carrickaness Sandstone is seen at Carricklongfield Quarry (ESCR Site 235). Much of this succession is also exposed in the River Blackwater (ESCR Site 238).
Triassic Sherwood Sandstone Group sandstones occur in a north - south strip on the eastern edge of LCA45, in faulted contact with Carboniferous to the east and west; in addition the sandstones lie unconformably and in faulted contact with Carboniferous Maydown Limestone in the east. They comprise red, purple and brown cross-stratified sandstones, siltstones with minor clay beds and partings. Exposed in (ESCR Site 267) Milltown stream, where Milltown Conglomerate and overlying Derrycreevy Sandstones are exposed.
Variscan and post-Variscan aged faults of many orientations cross the area.
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.
This is a drumlin dominated landscape forms part of the western Lough Neagh drumlin field and dominated by Late Midlandian till deposits laid down by ice that was centred on the Lough Neagh Basin. This approximately southward flow is reflected in the orientation of the drumlins. 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 it 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 addition to the numerous drumlins, the LCA also contains important sedimentological and morphological evidence of the deglacial processes that characterised the area. This is contained in the limited elements of two major deglacial complexes that are important scientifically and for their sand and gravel resources.
The Ballygawley Deltas Complex occupies a limited area of 1.7km2 in the west of the LCA. The complex as a whole 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, 22, and 23 with a most of the Complex in 44.
The Moneymore Glaciofluvial Complex in this LCA covers two small areas totalling 0.8km2. A small area in the north on the boundary with LCA 42 includes the Annahavil delta, while the other area is in the northeast on the border with LCA 64. Most of the complex is in LCAs 42 and 49. The Moneymore complex consists of discontinuous sand and gravel mounds and spreads in a north/south oriented belt in the Lough Neagh lowlands to the east of the Sperrin highlands between Moneymore, Coalisland and Dungannon. The landforms are dissected and consist predominately of ice-marginal and extra-marginal glaciolacustrine sequences superimposed upon the western Lough Neagh drumlin field. The stratified deposits formed in association with localised and variable palaeolake levels during the final deglacial stages of the region. Sediment supply was largely from a small, lowland residual ice-mass within the Lough Neagh basin and meltwater draining the eastern margin of the Sperrin highlands.
Key Elements
ASSI
100 BENBURB
The only interglacial peat deposit known in Northern Ireland containing fossiliferous evidence of an interglacial cycle. Assigned to the Gortian.
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 west of the LCA overlaps with LCAs 16, 17 and 44 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. For a fuller description of the Ballygawley Complex see LCA 44.
MONEYMORE glaciolacustrine COMPLEX
The Moneymore complex has good scientific value, and records the final ice-margins in the Lough Neagh lowlands during the deglacial period following eastward wastage of the ice from the Sperrin Mountains and Fintona hills into the lowlands. It also demonstrates the control shown by local (predominately drumlin) topography on waterbody ponding during the late stages of ice decay and is one of the few lowland locations recording the presence of fine-grained, glaciolacustrine sediments.
DONAGHMORE/ANNAHAVIL DELTA complex (MONEYMORE glaciolacustrine COMPLEX)
Landforms in the Donaghmore area are of importance in understanding the recent glacial history of Northern Ireland. A discontinuous ridge that extends for 3.5km south from Annahavil indicates that sediment was injected into a proglacial lake from several points along the western margin of the largely stagnant ice body centred on Lough Neagh. Later drainage of the lakes resulted in the dissection and subsequent subaerial erosion of the deposits. The linear, beaded morphology of ridge segments to the east of the main ridge suggests that they are eskers that supplied sediment to the ice front. This complex overlaps the boundaries between LCAs 42 and 45.
Other sites/units identified in the Earth Science Conservation Review
267 Milltown
Mesozoic. Sherwood Sandstone Group. Exposures of type localities for Milltown Conglomerate Formation and Derrycreevy Sandstone Formation.
15 BENBURB
Buried integrity.
234 Plaister Quarry
Carboniferous. Shallow exposure of very fossiliferous upper part of Maydown Limestone Formation. Fossils of Asbian age dominated by corals and brachiopods.
235 Carricklongfield Quarry
Carboniferous. Quality exposure of Carrickaness Sandstone Formation. Fossils include plant debris and corals.
238 River Blackwater
Carboniferous. Tyrone Group. Exposure of lithologies of Gorestown Mudstone Member. Abundant fossils.
246 Derraghadoan Pit
Exposure of lithologies of Rossmore Mudstone Formation.
247 Coalisland Brick Pit
Best exposure of Coal Measures strata of Westphalian age. Unfossiliferous mudstones with thin beds of sandstones. Fossiliferous bands with brachiopods and goniatites.
232 Drummond Quarry
Carboniferous. Most easterly outcrop of Bundoran Shale Formation. Abundant fossils include shelly fauna.
