Long Mountain Ridge 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 Long Mountain landscape character area comprises a long ridge of land, known as the `Long Mountain', that runs from Ballymoney in the north to Randalstown in the south. It has been created by the relative lowering of softer bands of Lower Basalt on either side of it which are now occupied by the Lower Bann and River Main valleys to the west and east respectively. The former extent of the Upper Basalt cover is indicated by outliers of these strata to the southeast of Portglenone and at Millars corner east of rasharkin. Despite variations in topography and character, the ridge reads as a single feature in the landscape and forms a distinctive skyline of a distinct rounded ridge orientated north-south with undulating side slopes and a broad rounded crest. The lower slopes on the northeastern flank of the LCA overlap with the Glarryford esker complex, where the ridge and mound topography adds interest to this area and to the low-lying, frequently bog-dominated, valley of the River Main. The south of the LCA also extends into the drumlin field that occupies the lower valley sides of the River Bann.

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.

Tertiary (Antrim Lava Group) stratigraphic succession (between 50 and 60 million years old)

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

This LCA extends south from Ballymoney to just north of Randalstown and forms the low rolling ground of mid Antrim and the Antrim Plateau. The geology comprises a mix of Tertiary igneous (lava flows, columnar jointed lava flows, ash-falls and red-weathered horizons or boles) formations in bedded, faulted and unconformable contact. Tertiary Lower Basalt Formation makes up 70% of the LCA with the remainder being the other formations in varying proportions.

The Lower Basalt Formation occurs in an extensive outcrop of the plateau of the LCA. They are extensively quarried for construction materials, especially roadstone.

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 covered by a thin mantle of till, but with significant drift free areas indicative of its overriding by late Midlandian ice moving northwards out of the Lough Neagh Basin. This flow has left numerous streamlined rock ridges orientated approximately S-N, as well as drumlins on the lower flanks of the ridge. In the northeast of the LCA there are limited deposits (1.1km2) of a deglacial sand and gravel complex. The Glarryford Esker/Outwash is an esker ridge and adjacent outwash spread located along the western side of the River Main at Ballymoney. Landforms record subglacial esker deposition, followed by proglacial outwash deposition, late in the deglacial cycle, within a tunnel valley cut into bedrock. Mounds of stratified sediment are probably the eroded remnants of more extensive gravelly spreads formed over ice or ice-rich sediment during final ice wastage.

Key Elements

Deglacial Complexes

GLARRYFORD ESKER/OUTWASH

The esker/outwash association is an important element in the reconstruction of the processes which pertained during the final stages of the Lough Neagh ice mass.