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 Antrim Plateau and Glens. This upland area is dominated by a series of structural plateaux that dip gently in towards the Lough Neagh Basin. Detailed topography is largely controlled by a succession of Tertiary basalt lava flows that define successive, large-scale steps within the landscape. The plateaux are separated from each other and their frequently dramatic margins are fretted by often fault-guided, steep-sided glens. Recession of the plateaux margins has exposed underlying Mesozoic strata and, in some areas, the Palaeozoic basement. The plateaux margins are typically characterised slope failures that range from large rotational landslides to individual blockfalls.
The edge of the Antrim basalt plateau is well defined by a steep scarp slope which wraps around and contains the north west edge of Belfast. The black basalt outcrops have a distinctive, sheer profile that is broken by a series of steep, wooded glens. The Hills are pitted with quarries and provide a dramatic contrast to the dense urban areas below. The dark basalt overlies a thin band of chalk, which forms a strong contrast in colour whenever it is visible. The lower escarpment slopes are a mixture of hummocky open pasture and stands of deciduous woodland on steeper slopes. The slopes are pitted with quarries that are prominent and exert a strong influence on landscape character and quality. The scarp slope is a prominent landmark of the area and its open character, distinct profile and high visibility render it particularly sensitive to change. Despite the degraded condition of some areas, it creates a distinctive setting for Belfast that merits conservation and management. The entire area is therefore classified as an `Area of Scenic Quality'.
Restoration of abandoned quarry workings will improve the visual appearance and landscape condition of the escarpment. Quarries in prominent positions are particularly important.
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 (mostly dykes), around 55 million years old |
| Tertiary - Lower Basalt Formation, about 55 million years old |
| Cretaceous - Hibernian Greensand & Ulster White Limestone, about 100 million years old |
| Jurassic - Waterloo Mudstone, about 100 million years old |
| Triassic - Penarth Group, about 205 million years old |
| Triassic - Mercia Mudstone Group, between 240 and 205 million years old |
The geology comprises a mix of faulted, Mesozoic sedimentary and Tertiary igneous rocks in faulted and unconformable contact. Triassic Mercia Mudstone Group and Tertiary Lower Basalt Formation each make up 40% of the LCA with the remainder being the other formations in varying proportions. The ESCR sites of Collin Glen, Crow Glen and Carr's Glen (265, 443, 442 respectively) expose the above succession.
Below the Cretaceous - Tertiary escarpment and in fault-bounded strips within the Tertiary basalts, low hills and low ground are underlain by soft sedimentary rocks of the Triassic Mercia Mudstone Group. The Mercia Mudstone Group is predominantly red-brown and unfossiliferous in the lower parts of the exposed succession, becoming grey-green, sometimes fossiliferous and sometimes carbonate-cemented toward the top. The beds form an aquiclude, soft and contain anhydrite. They have been quarried for brick clays here and elsewhere in last 200 years.
The Jurassic Waterloo Mudstone Formation crops out in a fault-bounded strip in the south of LCA112 below the basalt escarpment. These dark grey mudstones, grey to black shales and minor limestones are fossiliferous, containing ammonites and rare reptile remains.
The Cretaceous succession is found in a seriers of linear, fault-bounded exposures below the Tertiary basalts of LCA112. Basal fossiliferous sands and greensands are recorded overlain by the Ulster White Limestone Group is known from isolated exposures, old limekilns and marlpits in the area.
The Lower Basalt Formation occurs in an extensive outcrop along the northwestern edge of the LCA. They are extensively quarried for construction materials, especially roadstone. The clay with flints (a volcanic layer) occurs at the base and is exposed in Bellevue Quarry (ESCR Site 445). The Upper Basalts occur in isolated exposures in the west of LCA98. Dykes occur throughout the area: the most obvious are those seen cutting soft Mercia Mudstone group or in white limestone. These trend NW-SE but other dyke trends occur. The Ballygomartin Sill occurs in a stream at ESCR Site 444. NW - SE oriented faults dominate the outcrops of Mesozoic and Tertiary rocks and juxtapose all the above formations. NE-SW fault orientations occur in the southwest of LCA112.
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 clearly shows the drift free crestline of the basalt escarpment and also identifies large areas of landslip below the escarpment. 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 Antrim Plateau ice free and surrounded by encircling ice. Because of the oversteepening of the basalt escarpment by ice within the Lagan Valley, and the presence beneath the basalt and chalk of an impermeable, but weak layer of Lias and Rhaetic clays, the escarpment was inherently unstable following the removal of ice support from the Lagan Valley. Instability would have been further enhanced by the greater availability of groundwater as climatic conditions ameliorated. The collapse of large sections of the escarpment therefore represents a post-glacial adjustment in the landscape. This continues into the present-day, as the dropping down of large masses of basalt and chalk had the effect of bulging out the underlying beds, especially the Triassic marl. Localised areas of this disturbed material continue to intermittently creep downslope in West and North Belfast and are potentially unstable if oversteepened or undercut. Below the escarpment, the lower slopes are generally mantled with a cover of Late Midlandian till, which itself can be subject to slope failure when locally saturated.
Key Elements Sites/units identified in the Earth Science Conservation Review265 Collin Glen
Exposures of a range of Mesozoic rocks, especially Hibernian Greensands Formation and stratotype for Collin Glen Formation. Provides representative section for Lagan Valley.
442 Carr's Glen
Exposures of section through Mesozoic rocks - Mercia Mudstone Group and Waterloo Mudstone, Hibernian Greensand and Ulster White Limestone Formations.
443 Crow Glen
Exposure of sections through Mesozoic and Palaeogenic rocks - Mercia Mudstone Group and Hibernian Greensand, Ulster White Limestone and Lower Basalt Formations.
444 Ballygomartin Sill
Exposure of a basalt sill that is cut by a thin basalt dyke, indicating intrusion before end of Palaeogene dyke activity.
445 Bellevue
Best exposures in Greater Belfast area of 'Clay-with-Flints' horizon, interpreted as being of volcanic origin, at the base of the Lower Basalt Formation.