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 River Lagan flows in a winding, narrow channel across a very broad, flat valley to the west of Lisburn. The river is relatively inconspicuous towards the centre of the valley. The River Lagan itself drains eastwards from Silurian highlands immediately south of the Lagan valley along a topographic low underlain by Triassic sandstones, Cretaceous chalk and greensands. The valley continues between the southern margin of the Antrim Plateau basalts forming Divis Mt and Cave Hill (476m and 300m O.D.) and the Silurian highlands to the south. These hills form a northern backdrop to the LCA and it derives much of its landscape character from their presence. In the north of the LCA are elements of the Lagan Valley Glaciofluvial Complex, in the form of steep sided eskers, although much of their former extent has been destroyed through commercial aggregate extraction
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, especially dolerite, around 55 million years old |
| Tertiary Lower Basalt Formation, about 55 million years old |
| Cretaceous Hibernian Greensand and Ulster White Limestone, about 100 million years old |
| Triassic Mercia Mudstone Group, about 215 million years old |
| Triassic Sherwood Sandstone Group, about 240 million years old |
| Permian Belfast Group, about 280 million years old |
| Permian Enler Group, about 280 million years old |
| Lower Palaeozoic Gala Group, Gilnahirk Group, about 450 million years old |
The low hills of the southeastern side of the Lagan Valley form the southern boundary of LCA 108 and are composed of Lower Palaeozoic greywacke sandstones and shales. 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 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 easternmost part of LCA108 is composed of a small area of Permian Enler Group (comprising red-brown sandstones, conglomerates, siltstones). The Permian succession also forms low hills on the southeastern edge of the Lagan Valley. The Belfast Group comprises mudrocks with thin sandstone lenses resting conformably on Enler Group, a gradual transition between the two groups occurs. The topmost 1-4 metres of the Belfast Group at the type location in Belfast Lough, comprises the Magnesian Limestone Formation, a fossil-bearing limestone.
Triassic sandstones comprise red, purple and brown cross-stratified sandstones, siltstones with minor clay beds and partings. The sandstones are mostly soft and poorly-consolidated.
Below the Cretaceous - Tertiary escarpment and above the Sherwood Sandstones to the north and northeast of LCA108, low hills 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.
The Cretaceous succession is found in linear, fault-bounded exposures below the Tertiary basalt escarpment of LCA108 and comprises sandstones at the base and white limestones (chalks) above.
A dolerite plug (the core of an ancient volcano) occurs in the Sherwood Sandstone Group east of Moira and west of Flatfield in the west of the LCA.
The Tertiary basalt formations comprise a crudely-bedded succession of lava flows, columnar jointed lava flows, ash-falls and red-weathered horizons (or boles) that crop out in the far western and northwestern edges of the LCA. The basalts rest unconformably on the older formations. They are extensively quarried for construction materials in this area, especially roadstone.
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 underlain for the most part by Late Midlandian till laid down by ice that moved rapidly southeastwards across the area from a centre in the Lough Neagh Basin. This flow direction is confirmed by the extensive swarms of drumlins that are found in LCAs to the south of the Lagan Valley. However, in the east of the LCA there is also an extensive spread of glaciofluvial deposits. These form 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. In this LCA, the Complex is located in the extreme north and constitutes part of the Causeway End Esker that also runs through parts of LCAs 97 and 109. Most of the Complex can be found in LCAs 97, 106 and 107, with minor areas in LCAs, 62, 81 and 109.
The drift geology map also highlights the alluvial deposits associated with the floodplain of the present-day river Lagan.
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 Review452 Broad Water
Preserved largest glacial drainage channel in Lagan Valley, formerly linking Glacial Lough Lagan and Glacial Lough Neagh.