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 Slieve Croob Summits are a series of rounded summits formed from intrusive igneous rocks within a surrounding lowland area of sedimentary rocks. They stand out as distinct massive rocky summits with thin grass cover and shattered rocky screes. The land rises to a height of 534m at Slieve Croob. The lower slopes are marginal pasture divided by broken stone walls and small stone cottages, many of which lie derelict. The summits are an open, rugged landscape, with only a few conifer plantations. Drumkeeragh Forest, on the slopes of White Hill, is the largest. Few roads cross the landscape but viewpoints such as Windy Gap allow panoramic views over the surrounding lowlands, as well as across to the Mourne Mountains. The prominent ridge lines are visible from miles around, particularly those formed by the Slieve Croob/Slievenisky complex and Slievegarran. Much of the area falls within the Mourne Area of Outstanding Natural Beauty; its scenic qualities are therefore recognised and merit continued conservation.
Pre-Quaternary (Solid) GeologyThe 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 - various late intrusives, about 50 million years old |
| Tertiary - Mourne Granites - about 55 million years old |
| Tertiary - early intrusives |
| Late Caledonian Newry Complex Intrusives, about 420 million years old |
| Lower Palaeozoic - Ordovician (predominant) - Gala Sandstone, about 450 million years old |
Comprises 60% Gala Group (Lower Palaeozoic), 20% Mourne granite and the remainder being a variety of Caledonian and Tertiary intrusives.
Lower Palaeozoic greywacke sandstones and shales dominate the northern and western area. The greywackes are of sandstone grade and vary from a few centimetres to a few metres in thickness with a large proportion of rock fragments. The greywackes are commonly quarried as a source of aggregate; they are interbedded with thinner beds of siltstone or mudstone, commonly arranged as fining-up cycles. Minor conglomerates and volcanic ash-beds occur.
Slieve Croob, in the southern part of LCA87, is composed of Caledonian (probably Devonian age) Newry Complex diorite (an igneous rock like a granite). The eastern outcrop contact between Gala Group greywackes and Slieve Croob diorites has formed hornfels metamorphism of the country rock greywackes, exposed at Slieve Croob itself (ESCR Site 414).
Tertiary-aged dolerite and felsite dykes occur throughout the area. Certain trends dominate. In the Lower Palaeozoic Gala sediments to the north of LCA87, these dykes are either NW-SE or NE-SW. Far fewer dolerite dykes are observed cutting the Mourne granites, suggesting that the main emplacement of the Tertiary dykes was pre-granite.
Mourne Granites (Tertiary)
The Mourne Granites were emplaced in successive injections at two centres: LCA87 covers the the first granite (G1) of the eastern centre. G1 comprises a felspathic, hornblende granite.
Quaternary (Drift) GeologyNorthern 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.
Davies and Stephens (1978) consider that the final stages of ice-wasting in the east of Northern Ireland probably involved wide scale stagnation, downwasting and withdrawal inland towards the Lough Neagh lowlands (p.176). This would have left upland areas such as the Slieve Croob as ice free areas surrounded by encircling ice. The drift geology map for this LCA confirms the drift free character of the upland area of Slieve Croob that was overridden by Late Midlandian ice. However, around the base of the upland, that same ice has left a depositional record of tills that reach upwards into the massif along its valleys. Although upland areas were scoured clear by the ice it is possible to find within the landscape pockets of deeply weathered granodiorite that can exceed 10m in depth (Smith in Whalley et al. 1985). This sandy, disaggregated material has previously been described as a fossil soil dating from pre-Quaternary times, when the climate over what is now Northern Ireland was tropical in nature. However, current thinking on these 'arenaceous' deep weathering profiles suggests that they can form under temperate climatic conditions, especially if drainage conditions are acidic. It is possible therefore that the material might represent deep weathering during an interglacial phase or phases, or possibly under warmer conditions during and since the Midlandian cold period. In the absence of any datable material capping the profiles - as with the inter-basaltic beds of the Antrim Plateau - it is unlikely that it will be possible to attach a definitive date to these profiles
Key Elements Sites/units identified in the Earth Science Conservation Review414 Slieve Croob
Caledonide-Igneous. Exposures of contact metamorphosed Silurian country rock. Access to hornfels and fused sediments.
AONBsAlmost all of this LCA lies within the Mourne AONB (1986). This designation is indicative of the scenic quality of the landscape.