Eastern Binevenagh Slopes 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 on the northeastern flank of 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 Eastern Binevenagh Slopes character area is the sloping basalt plateau to the east of Binevenagh. The area is drained by numerous branching streams flowing within narrow, well-defined valleys or in steep gullies on higher land. The surface of the basalt plateau is rolling, with a series of broad ridges aligned northwest to southeast. They have an irregular, slightly crumpled surface. This is an area of relatively poor quality farmland; mostly rough pastures with acidic soils and patches of gleyed and marshy soils. Fields are generally small on steeper land and have geometric, angular shapes. Quarrying is a pressure in some areas, but has a relatively local influence and the associated scrubby vegetation seems to screen most sites. The landscape can therefore be summarised as one of low rolling plateau, falling gradually from basalt uplands eastwards towards the River Bann, with fairly steep, narrow glens; gullies on higher land to west. 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. This is most evident at Downhill, where there is historical evidence for the retreat of the coast towards the cliff top Mussenden Temple.

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)

This LCA is dominated by Upper Basalt Formation: a crudely-bedded succession of lava flows, columnar jointed lava flows, ash-falls and red-weathered horizons (or boles). Two NW-SE Tertiary dolerite dykes are mapped at the northern end of LCA38.

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 primarily a complex of bare rock and till deposited during the drumlin readvance on the flanks of Lough Neagh ice moving northwards along the Bann valley. This is evidenced by southeast to northwest oriented streamlined rock ridges and drumlins. The northwest margin of the LCA lies just outside the boundary of the Armoy moraine, that marks the southern limit of a late southwards advance of Scottish ice. The nature of this ice marginal environment is indicated by the presence of glaciofluvial deposits and ice directed meltwater channels that were also left during deglaciation as the Lough Neagh ice eventually retreated southwards at the end of the Midlandian. For further information on the Armoy Moraine, see LCAs 54 and 56.

Key Elements

ASSI

The northern margin of the LCA touches on the MAGILLIGAN ASSI (068), see LCA 35.

AONBs

A very small area of the north of the LCA lies in the North Derry AONB (1966), whilst a small area in the south overlaps the Sperrin AONB (1968).