Cookstown Farmlands 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 Cookstown Farmland is a drumlin landscape to the west of Lough Neagh. The drumlin landform is relatively shallow in the Cookstown area, but becomes steeper and more pronounced on the higher land to the west near Beaghmore and towards Slieve Gallion to the north west. Further to the east, the landform flattens towards the floodplains of the Ballymully, Ballinderry and Killymoon Rivers. The drumlins are less well defined than others elsewhere in the Province, giving small landscape elements, such as hedgerow trees and individual buildings greater prominence. The floodplain landscapes have a distinctive character, with extensive wet woodlands and embanked roads. A key element in the south of the LCA is part 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.
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 and sills, about 60 million years old |
|---|
| Cretaceous - greensands and limestones, about 100 million years old |
| Triassic - Mercia Mudstone Group and Sherwood Sandstone Group, between 240 and 210 million years old |
| Carboniferous - Iniscairn, Ballyness, Clogher Valley, Carboniferous Limestone (undifferentiated), Rockdale, Derryloran, Rossmore, Millstone Grit (undifferentiated), Coal Measures (undifferentiated) - between 350 and 290 million years old |
| Devonian - Shanmaghery, Cross Slieve Group, 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 succession at Ballysudden is in ASSI 133.
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 most of it is dominated by till deposits resulting from the drumlin readvance. Away from the lake shore, the landscape of the western half of the LCA is shown in Edwards (1980) to be dominated by large numbers of E-W orientated drumlins that form part of the Western Lough Neagh Drumlin Field. 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.
Within this drumlin dominated landscape, the south and east of the LCA contain significant spreads of glaciofluvial deposits formed during the deglaciation of the region and the wasting of the Lough Neagh ice sheet. In particular, The Moneymore Glaciofluvial Complex covers 5.6km2 of this LCA as a small area in the extreme north and along its southeastern margin with LCA 45 where it includes the Annahavil delta. The other major expanse of the complex is in LCA49, with limited ouliers in LCA 45. 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 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.
The drift map also highlights the alluvial deposits associated with the present-day drainage system.
Key Elements
ASSIs
113 BALLYSUDDEN
Ballysudden contains some of the finest Carboniferous limestone palaeokarst features exposed anywhere in the British Isles. These limestones, over 100m thick and some 335 million years old, formed in a shallow marine environment and are rich in fossils such as brachiopods and corals. Emergence and exposure of the former limestone seabed to terrestrial conditions allowed soil development and solution of the limestone to commence. This resulted in the formation of potholes and other solution hollows which can attain substantial sizes within the site. Palaeontological investigations of the infill may offer the rare opportunity to assess a Carboniferous terrestrial environment with the possibility of macro fossil remains.
097 LITTLE RIVER (ca 25%)
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
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
Silurian
Little River Group
Mudstones and siltstones with graptolite fossils, exposed in the Little River ASSI (097) and thus the type section of this formation. This is the type locality for over 60 fossil species and one of only two localities in N.Ireland where Silurian shelly fauna can be found.
Permian
Enler and Belfast Group
These NW-SE striking calcareous mudrocks with thin sandstone and anhydrite lenses occur in a faulted area in the south-east of LCA42, resting unconformably upon Carboniferous (Upper Palaeozoic) Rockdale limestones. The calcareous mudrocks are the equivalent of the Magnesian Limestone: two unique exposures occur in this LCA: at Tullyconnell and at Grange ESCR sites 293 and 165 respectively.
Triassic
Sherwood Sandstone Group
These north - south striking sandstones lie unconformably Carboniferous sediments, cropping out as a c.5 - 8km-wide area of low ground in the northeast of LCA42 (~20% of total area). They comprise red, purple and brown cross-stratified sandstones, siltstones with minor clay beds and partings. Outcrops occur in Draperstown Quarry (ESCR Site 268) and Lissan Water (ESCR Site 269).
Tertiary
Coagh Conglomerate
The base of the Tertiary basalt succession in this area is represented by the Coagh Conglomerate, a unit not seen elsewhere. It comprises a fluvial conglomerate with basalt and flint clasts with laterite debris. Thickness unknown but exceeds tens of metres. This is an unusual deposit, not seen in many other locations.
The Sherwood Sandstone in intruded by a discontinuous sill (north - south) in its upper beds west of Cookstown. A few Tertiary dykes are recorded - in the north and south central areas.
Caledonian, Variscan and post-Variscan aged faults of many orientations cross the area. The most significant is the Elagh Fault, a post-early Tertiary, reactivated Variscan, NE-SW oriented structure that juxtaposes Cenozoic-Mesozoic with Carboniferous. A major NE-SW trending syncline in the southwest of the LCA preserves Clogher Valley Formation in the core of an arcuate outcrop of Ballyness Formation.269
Draperstown Quarry
Mesozoic - Triassic. Exposure of Ballyloughan Formation, providing evidence for the depositional environment of Sherwood Sandstone Group: important for hydrogeological and hydrocarbon exploration.
