Belfast/Lisburn 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 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 Belfast/Lisburn LCA encompasses the Belfast and Lisburn urban areas, together with their broader landscape setting. It is defined by the steep ridges and escarpments that enclose the Lagan Valley at the head of Belfast Lough, and derive much of its landscape character from its setting in this fault-guided depression. 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 occurs between the southern margin of the Antrim Plateau basalts forming Divis Mountain and Cave Hill (476m and 300m O.D.) and the Silurian highlands to the south. Belfast is mostly contained within the valley, although urban development has spread along the narrow coastal strips to the north and south of the Lough and inland along the Lagan and Enler Valleys. The town of Lisburn is also sited on the River Lagan but is further upstream and enclosed by slightly lower slopes. There are long views over both urban areas from the surrounding upland landscapes. The steep escarpments and ridges on the margins of the city provide a series of landmarks. The Belfast Basalt Escarpment to the north has a rugged, untamed character; Carmoney Hill, an outlier to the north west of the basalt escarpment, dominates the landscape setting of Newtownabbey; the Craigantlet Escarpment on the fringes of the Holywood Hills encloses East Belfast and the slopes of the Castlereagh Escarpment provide a continuous backdrop to the urban districts of South Belfast and Castlereagh. A key geomorphological features of the Lagan valley is its extensive suite of glaciofluvial deposits. These sand and gravel deposits have historically been used as the preferred sites for building and today underlie the most of the urban areas in the valley. No active quarries exist. However, the deposits remain a vital source of evidence for understanding the late-glacial history of this area and of Northern Ireland. Unfortunately, The urban areas of Belfast and Lisburn largely obscure most of the topographic diversity added by the deposits and the M1 motorway dissects and breaks the visual continuity of the arcuate cross-valley ridges which occur along the axis of the Lagan valley. The northeast of the LCA is marked by the largely fault-guided depression of the Dundonald Gap, which cuts through the Silurian hills and probably acted in the past as a glacial meltwater channel linking the Lagan Valley to Stranford Lough.

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.

Stratigraphic Table (youngest rocks at the top of the table)

The northeastern area between Belfast Lough and the Craigantlet Hills encroaches upon the Lower Palaeozoic outcrop. These comprise the greywacke sandstones a few centimetres to a few metres in thickness with a large proportion of rock fragments and a fine-grained matrix. The latter are greywackes with minor shale beds exposed in ESCR Site 454, Cregagh Glen. Both units are commonly quarried as a source of aggregate. Minor conglomerates and volcanic ash-beds (or bentonites) occur.

The red conglomerates, sandstones, red grey mudstones and laminated silty limestones of the Holywood Group succession (Lower Carboniferous) are subdivided into a number of formations, largely discernable from boreholes sunk in the Belfast Harbour area. The coastal outcrop strip is dominated by the interbedded siltstones, mudstones and limestones of the Ballycultra Formation, which is found in faulted contact with Lower Palaeozoic strata to the north and Triassic to the south.

The Permian comprises red-brown sandstones, conglomerates, siltstones, preserved beneath the Dundonald Sill at ESCR Site 455, Moyard. The topmost 1-4 metres of the sequence comprises the Magnesian Limestone Formation, a dolomitic, fossiliferous limestone.

Triassic sandstones comprise red, purple and brown cross-stratified sandstones, siltstones with minor clay beds and partings.

Below the Cretaceous - Tertiary escarpment and above the Sherwood Sandstones to the north and northeast of LCA97, low hills are underlain by soft sedimentary rocks of the Triassic Mercia Mudstone Group, the top part of which is exposed at the type location in this LCA, [ESCR Site 265] of Collin Glen Forest Park.

The Jurassic Waterloo Mudstone Formation, crops out in the north of the LCA.

NW-SE trending dolerite dykes occur throughout the area and are especially obvious in the northern area within the soft Mercia Mudstone Group. The Dundonald Sill is seen at Moyard (ESCR Site 455).

Tertiary-aged 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. Basal clay with flints exposed in Bellevue Quarry (ESCR Site 445).

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 of the LCA reveals a complex overlay of late- and pot-glacial deposits. Around the head of Belfast Lough it clearly shows the land 'reclaimed' by landfill - although the boundary of this must be constantly changing as more infilling takes place. It also shows the alluvial deposits associated with the present-day Lagan and its tributaries. Beneath all of this is a cover of late Midlandian till that was associated with the large ice mass that was centred on the Lough Neagh Basin. However, of greatest geomorphological interest is the extensive deglacial complex that occupies much of the Lagan Valley floor and extends northeastwards towards Comber via the 'Dundonald Gap'. The Lagan Valley Deglacial Complex is a discontinuous belt of glaciofluvial and glaciolacustrine deposits 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. Other significant elements of the Complex can be found in LCAs 106 and 107, with minor areas in LCAs 62, 81, 90 108 and 109.

Key Elements

Deglacial Complexes

LAGAN 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.

Other sites/units identified in the Earth Science Conservation Review

265 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.

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.

454 Cregagh Glen

Extensive exposures of large section of Gilnahirk Group, showing alterations of arenaceous and argillaceous beds - some with purple, lilac or green coloration.

455 Moyard

Condensed exposure of one of the few outcrops of Carnamuck Formation of the Enler Group, below Dundonald Sill.

AONBs

A significant area in the south of this LCA comprises the Lagan Valley AONB (1965). This designation is indicative of the scenic quality of the landscape.