Divis Summits 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 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 Antrim basalt plateau ends in a series of broad, rounded summits overlooking Lisburn and Belfast. The principal summits of Black Mountain, Squires Hill and Cave Hill reach up to 400m, with Divis standing at 478m. They generally have gentle slopes, with some gullies and abrupt, steep slopes in places. However, the summits along the edge of the basalt escarpment have much steeper slopes that plummet towards Belfast. Between the summits, the upper plateau has extensive areas of shallow, partially waterlogged moss that is surrounded by areas of marginal farmland. The landscape of the Divis Summits is generally in poor condition, particularly on the summits and in areas where there has been a history of mineral extraction. The basalt summits and steep escarpment slopes are particularly sensitive to change, as they form the backdrop to the urban areas of Lisburn and Belfast and because they are relatively exposed. The quarry sites are often in extremely prominent locations and their restoration should be considered a priority. Hazelwood Area of Scientific Interest occurs within this character area, forming a particularly sensitive part of the landscape.

The restoration of abandoned quarry sites will improve views to this landscape; priority should be given to those in the most prominent positions on the escarpment slopes and those in the vicinity of important archaeological sites.

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 geology comprises a mix of Mesozoic sedimentary and Tertiary igneous rocks in faulted and unconformable contact. Tertiary Lower Basalt Formation makes up 85% of the LCA with the remainder being the other formations in varying proportions.

The ESCR Sites of Carr's Glen and Crow's Glen (442 and 443) expose the above succession.

Below the Cretaceous - Tertiary escarpment, the low ground is underlain by soft sedimentary rocks of the Triassic Mercia Mudstone Group. 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 fault-bounded strips along the east of LCA111 below the basalt escarpment. These dark grey mudstones, grey to black shales and minor limestones contain ammonite and rare reptile fossils.

The Cretaceous succession is found in a series of linear exposures below the Tertiary basalts of LCA111. Basal fossiliferous sands and greensands are overlain by the Ulster White Limestone Group.

The Tertiary Lower Basalt Formation occurs in an extensive outcrop of the plateau of the LCA. They are extensively quarried for construction materials, especially roadstone. Ash-falls within the Lower Basalts are recorded: one such unit occurs in a small part of thesouthwest of LCA111.

The two Tertiary-aged basalt formations comprise a crudely-bedded succession of lava flows, columnar jointed lava flows, ash-falls and red-weathered horizons (or boles). In the northern part of LCA111, these two formations are separated by red, palaeoweathered beds and ashfalls of the Interbasaltic Formation.

NE-SW and NW-SW oriented faults occur in the eastern parts of the LCA, where they juxtapose all the above formations. Dykes occur throughout the area: the most obvious are those seen cutting the Lower Basalts in the northeast of the LCA. These trend NW-SE but other dyke trends occur.

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 for this LCA shows it to be underlain for the most part by Late Midlandian till laid down by ice that moved rapidly across the area from a centre to the west in the Lough Neagh Basin. The exceptions to the drift cover are the exposed bedrock areas associated with the crest line of the plateau. There is also an extensive drift free, stepped area overlooking Mallusk and Carnmoney to the north that clearly illustrates the structural control exerted by the successive lava flows. 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. This retreat of the ice was probably allowed the deposition of a very small area of glaciofluvial deposits in the northwest of the LCA in what must have been an ice marginal location.

Key Elements

Sites/units identified in the Earth Science Conservation Review

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.