Armagh Drumlins 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 Uplands and Drift Covered Lowlands of Down and Armagh. The generally subdued relief associated with the underlying basement complex of highly folded Palaeozoic strata provides the unity of this region. Relative relief is provided in the north by the Silurian hills that overlook the lower Lagan Valley, The Newtownhamilton Plateau in south Armagh, the Caledonian igneous complex of Slieve Croob and the structural depression that underlies and defines Strangford Lough. Below ca 350m, there is an almost complete mantle of drumlins forming an internationally acknowledged type example of a 'drumlin swarm'.

The Armagh Drumlins cover an extensive area of rolling north - south orientated drumlins. They are overlooked by the Carrigatuke Hills to the south and fall towards the Loughgall Orchard Belt and fringes of Lough Neagh to the north. The area is drained by numerous small winding streams that are frequently tributaries of the Callan River. Occasional loughs and sedgy mosses occupy the hollows between drumlins. The landform becomes progressively lower and the drumlins more pronounced to the north. River and stream valleys, loughs and mosses are sensitive to changes in water quality and water table, the latter being easily affected by development.

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)

Comprises a hugely complex area of geology with rocks from Lower Palaeozoic through to Tertiary in complicated unconformable and fault-bounded contact.

Carboniferous

Annaclare Group breccias, sandstones and siltstones seen at Killuney Stream -ESCR Site 293. Armagh Group limestones and shales with Drumman More Sandstone (exposed in Ballynahone River, ESCR Site 240), Maydown Limestone, Carrickaness Sandstone and Blackwater Limestone formations above and partially interbedded with the Armagh Group limestones (exposed in Rock Road Quarry, highly fossiliferous [corals] ESCR Site [241]). The Gorestown Mudstone Member is exposed in the river and disused quarry of the River Blackwater (ESCR Site 238). Carboniferous strata are also observable at Drumarg Cliff and Annacramph Lane (ESCR Sites 242 and 243, respectively.

Permian - Enler and Belfast Group

The basal Permian breccias (Drumarg Conglomerate) occurs at the type section and most westerly occurrence of this rock type (Drumarg, ESCR Site 262).

Triassic - Sherwood Sandstone Formation

The Milltown Conglomerate and Derrycreevy Sandstone members occur at (ESCR Site 267) Milltown.

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 an area largely covered by Late Midlandian till deposited by ice that moved southwards across the area from an ice centre in the Lough Neagh Basin. Morphologically, the landscape is dominated by 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.

In the extreme northeast the LCA overlaps a small area of the Poyntz Pass glacial drainage channel near Tandragee. This channel formed during the deglaciation of the Lough Neagh lowlands, when there was a period when downwsting ice occupied the Lower Bann valley and prevented the northwards drainage of the proto-Lough Neagh. Lake levels then rose until an alternative outlet was found to the south via Pontz Pass and Newry to Carlingford Lough (Davies and Stephens 1978). McCabe and Hirons (1986) has described this drainage channel as having similarities to a tunnel valley system, in which sand cored drumlins occur within the channel system itself and are flanked by large rock drumlins west and east of the channel.

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 recent years, researchers have also identified in this area a great many subglacial diamict (till) ridgesor 'rogen moraines' that lie transverse to the southwards Late Midlandian ice flow. Some of ridges were streamlined and overprinted by subsequent drumlin development, while others remained unaffected. This combination of subglacial bedforms is used to suggest that during the last deglacial phase ice masses were highly mobile and that flow was episodic due to variations in the subglacial thermal regime. One consequence of this was a high sediment flux to the ice margins that in turn generated significant glaciofluvial complexes. Within this LCA no extensive spreads of glaciofluvial deposits are mapped, but there are isolated sand and gravel deposits associated with local drainage lines that are now delineated by alluvial deposits.

Key Elements

Karst Features

Drumarg

To the east of Drumarg Cliff (Drumarg townland) is a second, lower (1 - 3m), limestone bluff. This outcrop contains a series of cave entrances with the main passage from each sub-parallel and bearing roughly eastwards. The ground overlying the passage series is highly disturbed with apparent doline features, limited 'pavement' and one vertical shaft present.

Other sites/units identified in the Earth Science Conservation Review

262 Drumarg

Mesozoic. Stratotype for Drumarg Conglomerate Formation. Most westerly exposure of Basal Permian Breccias In Northern Ireland.

267 Milltown

Mesozoic. Sherwood Sandstone Group. Exposures of type localities for Milltown Conglomerate Formation and Derrycreevy Sandstone Formation.

238 River Blackwater

Carboniferous. Tyrone Group. Exposure of lithologies of Gorestown Mudstone Member. Abundant fossils.

293 Killuney Stream

Exposure of Sub-Carboniferous unconformity, overlain by strata representative of Killuney Conglomerate Formation and Retreat Siltstone Formation.

240 Ballynahone River

Carboniferous. Exposure of Drumman More Sandstone. Stratotype of Ballynahone Micrite Formation of Annaclare Group. Foraminifera fossils in lowest limestone section.

241 Rock Road Quarry

Carboniferous. Exposures of 2 principal limestone lithologies of Asbian succession within Armagh Group. Concentrated accumulation of coral fossils.

242 Drumarg Cliff

Carboniferous. Shallow water limestones, with pedogenic horizons and associated disruption of limestone-shale sequence. Abundant marine fossils such as corals and brachiopods.

243 Annacramph Lane

Carboniferous. Exposure of sequence of interbedded sandstones, limestone boulder breccias and limestones. Fossils of algae and foraminifera occur.