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 Castlereagh Slopes are the steep slopes of the north western margins of the Castlereagh Plateau, to the south east of the Belfast/Castlereagh urban area. The prominent ridge has a smooth, rolling landform and a steep gradient. It averages 130m but at Braniel Hill, reaches an approximate height of 170m. It is deeply dissected by steep glens that are generally well-wooded. The broad slopes of the ridge are divided into a patchwork of undulating pastures and hedgerows. The entire ridge is prominent in views from the lowland and is an extremely sensitive landscape in visual terms, as it forms a backdrop to views from Belfast and Lisburn.
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.
Stratigraphic Table (youngest rocks at the top of the table)
| Tertiary - intrusives, around 55 million years old |
| Permian - Enler Group, about 280 million years old |
| Lower Palaeozoic - Gala Sandstone, Gilnahirk Group, Moffat Shale, between 490 and 450 million years old |
The Lower Palaeozoic greywackes are of sandstone grade and vary from a few centimetres to a few metres in thickness with a large proportion of rock fragments and a fine-grained matrix. 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 (or bentonites) occur. The Gilnahirk Group strata are well-exposed in Cregagh Glen (ESCR Site 454) where purple, green and lilac coloured greywackes and shales area seen.
Within such fault zones, slivers of Moffat Shale (stratigraphically equivalent to Gala Group) have been brought into juxtaposition. One such NE-SW striking fault in the far south of the LCA occurs with Moffat Shale. The Gilnahirk Group is also seen in Purdy's Burn (ESCR Site 456) where a lamprophyre dyke occurs.
The Permian comprises red-brown sandstones, conglomerates, siltstones.
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.
The drift geology map for this LCA shows it to be predominantly underlain by Late Midlandian till associated with the large ice mass that was centred on the Lough Neagh Basin. This ice flowed southeastwards from an ice divide that lay approximately SW-NE along the line of the north Belfast Hills. Evidence for this flow direction is found in the orientation of the numerous drumlins that make up much of the landscape to the south of the LCA. Along the northwestern margin of the LCA, the drift geology map also shows a limited expanse of glaciofluvial deposits. These are part of the Lagan Valley Deglacial Complex. This is a discontinuous belt of glaciofluvial and glaciolacustrine deposits that occurs for 40km along the axis of the Lagan valley from Belfast WSW to Aghalee, Co. Antrim. The complex consists of four main elements; (1) poorly exposed deltaic sands which underlie most of south Belfast; (2) steep-sided esker remnants at Lisburn; (3) flat-topped cross-valley ridges with associated feeder channels at Drumbeg, Sandymount and Hillhall; (4) a flat glaciofluvial outwash spread at the Maze. Kettle-hole depressions and meltwater channels also occur occasionally.
Key Elements Deglacial ComplexesLAGAN VALLEY DEGLACIAL COMPLEX,
The Lagan Valley Deglacial Complex is highly important in understanding the complexity of deglacial processes. Streamlined landforms along the margins of the valley and glacially moulded bedforms indicate ice advance and episodes of fast ice flow from the west. Glaciolacustrine deposits indicate that during initial deglaciation the lower valley contained an ice-dammed lake, probably impounded by Scottish ice in outer Belfast Lough. A lobe of Irish ice located in the valley, related to ice pressure from the Lough Neagh Lowlands contained subglacial conduits now recorded by eskers that probably supplied sediment to the Malone deltaic sands that now underlie most of south Belfast. The phased retreat of the ice lobe further westward is recorded by cross-valley ice-contact ridges. During the final deglaciation, drainage was to the west, indicating a reversal in the drainage gradient probably due to isostatic depression of the Lough Neagh Lowlands during the last glacial cycle.
Sites/units identified in the Earth Science Conservation Review454 Cregagh Glen
Extensive exposures of large section of Gilnahirk Group, showing alterations of arenaceous and argillaceous beds - some with purple, lilac or green coloration.
456 Purdy's Burn
Extensive exposures of greywacke turbidites of Gilnahirk Group and Strangford Group. Also Exposure of Caledonian lamprophyre dyke.
AONBThe southern part of the LCA lies within the Lagan Valley AONB (1965). This designation is indicative of the scenic quality of this part of the landscape.