Binevenagh 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 North Derry Uplands and Sperrin Mountains. This region has a composite geological structure. In the north, the North Derry Plateau is wholly developed on basalt and defined by a steep, unstable escarpment to the west and a set of structural benches dipping gently to the east. Southwest of this plateau land, and beyond the Glenshane Pass, schists and quartzites form the rounded, whaleback ridges of the High and Low Sperrins. The incised, steep-sided valleys of rivers such as the Glennelly and Owenkillew accentuate the southwestwards, Caledonian structural trend of the Mountains. Late Glacial ice recession from around the mountains and the creation of temporary ice-dammed lakes has left valley floors and slope foot zones mantled in thick, complex glaciofluvial deposits. Northwest of the Sperrins is a dissected block of country underlain by schists that forms the Loughermore-Altahullion hills and the Middle Faughan basin.

The escarpment of Binevenagh is a dominant feature of the landscape to the east of the Roe Basin. It is a high cliff, with a sequence of steep summits that form striking local landmarks with distinctive slope profiles. Typically, a steep, even slope leads up to a near vertical cliff face of grey basalt. The summits average 400m and are close to the cliff face, emphasising the massive scale of the plateau and the structure of its broken edge. The basalt has acted as a protective cap to the softer Cretaceous, Jurassic and Triassic rocks that lie below and these are exposed towards the foot of the escarpment at the fringe of the plateau. The surface of the upper plateau is gently rolling, with numerous tiny burns and ditches. The basalt escarpment is characterised by large rotational landslides in which sections of cliff rotate forward at the base. This induces the top of the rotating block to drop and tilt backwards in towards the plateau. The front slopes of these blocks can then be sublect to a variety of smaller scale slumps and at sites such as Binevenagh, this sometimes creates a more complex, craggy slope profile and there are deep gullies on the steep escarpment slopes. The basalt plateau gradually decreases in height towards the east, where there is a gradual transition from moorland to farmland. This is an expansive moorland landscape, dominated by a large-scale mosaic of rough pasture, bog and conifer plantations. Rocky outcrops and patches of stunted scrub contribute to a wild, unstructured landscape character, although extensive dark conifer plantations introduce a more regimented element; they generally have hard, geometric edges which cut across the grain of the landform, subdividing the otherwise open moorland. The northern coast of the LCA comprises cliffs of multiple basalt lava flows. Between these are structurally weaker palaeosol horizons of weathered basalt that encourage localised collapse as the wear back and undermine the overlying basalt

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. The older formations can be upside down (tectonically inverted).

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

The Dalradian succession comprises metamorphosed sediments, igneous intrusions (metabasites) and volcanic green beds. The Carboniferous comprises conglomeratic sandstones and sandstones unconformable upon Dalradian. The Drumard Sandstone Member crops out in Barony Glen (ESCR Site 256). Triassic - Sherwood Sandstone Formation crops out in the stream at Curraghlane (ESCR Site 270). The Tircrevan Sandstone Member occurs in this section and is exposed at the possible ASI of Tircrevan Burn (ESCR Site 273).

Upper Basalt Formation crops out at Binevenagh (ASSI 212).

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 reflects its upland nature and comprises large drift-free areas of ice scoured bedrock. Between the bedrock areas there is an overlay of Late Midlandian till resulting from ice that moved northwards from a centre in the region of Lough Neagh. The deglaciation of the area was associated with the deposition of small patches of glaciofluvial sand and gravel in the upland valleys and with the large area of landslip that is mapped below Binevenagh. The northern margin of the LCA touches on the Magilligan Foreland (LCA 35) and comprises a mix of aeolian sand and raised beach deposits. Much of the highest uplands carry a peat cover.

Key Elements

ASSIs

The northern edge of the LCA touches on Magilligan ASSI (068).

212 BINEVENAGH

The area is of special scientific interest because of its geological and geomorphological features, its cliff vegetation, grasslands (in particular herb-rich calcicolous grassland), bryophyte-rich scree, rare plants, bryophytes and fungi. Binevenagh is an important outcrop of basalt, providing a fuller understanding of the development and age of the Antrim Lava Group as a whole. The well-defined flow units and pahoehoe surfaces are among the best examples seen anywhere within the basalt plateau. Mass movement during late- or post-glacial times has resulted in large slipped masses of basalt. The cliffs at Binevenagh have a unique assemblage of arctic-alpine plants and bryophytes.

Other sites/units identified in the Earth Science Conservation Review

270 Curraghlane

Mesozoic. Good exposure of representative lithologies of Triassic Sherwood Sandstone Group.

273 Tircreven Burn

Mesozoic and Palaeontological. Exposures of type section of Tircreven Sandstone Member of Waterloo Mudstone Formation. Also exposures of Cretaceous strata and outcrop of Lias - abundant reworked Jurassic fossils, especially vertebrate ones.

256 Barony Glen

Carboniferous. Exposure of Drumard Member strata. Evidence of soil formations in some limestone bands.

AONB

Nearly all of the LCA lies within the North Derry AONB (1966). This designation is indicative of the scenic quality of the landscape.