Garron Plateau 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 Garron Plateau is part of a `table' of Upper Basalt which stretches from central Ballymena to the coast at Garron Point, where dramatic cliffs with a distinctive stepped profile plunge into the sea. A distinctive landmark is Lurigethan and its Promontory Fort. The area is similar to the Larne Basalt Moorland in geology, land use and settlement, but its relief is more uneven, with rocky outcrops, deeply incised water courses and steep cliffs. The elevation is higher with summits reaching over 400 m in many places. Carncormick, Mid Hill and Soarns Hill are the highest mountains in the area. The uneven landform harbours many upland loughs and reservoirs. The condition of this landscape, which falls within the Antrim Coast and Glens Area of Outstanding Natural Beauty, contributes to its scenic value. The most sensitive areas occur in the wildest and highest summits where valuable peatland habitats have been designated as the Garron Plateau ASSI in recognition of their scientific value. Two other areas on the eastern side of the plateau hold ASSI designations (Blackburn ASSI and Gortnagory ASSI). Blackburn is of particular interest as it provides evidence of active dissolution of the Ulster White Limestone underlying the plateau basalts.

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)

This LCA extends forms the hills and plateau inland from the north Antrim coast at Garron Point - including Collin Top and Garron Plateau (ASSI 067). LCA122 contains igneous and sedimentary rocks of Triassic/Cretaceous to Tertiary age, Tertiary basalts rest unconformably and in faulted contact with Cretaceous greensands and limestones.

Below the Cretaceous - Tertiary escarpment in the far northern tip of LCA 122, the low ground is underlain by soft sedimentary rocks of the Triassic Mercia Mudstone Group.

The Cretaceous succession forms a limited outcrop strip to the north of LCA122. Above the basal greensand occur the limestones of the Ulster White Limestone Group. These bedded limestones are 30 - 80m thick, bedded, fossiliferous and contain chert (or flint) nodules and beds; the sediments were laid down in a tropical sea some 90 million years ago. The limestones are quarried extensively for lime and aggregate: the extensive Glenarm limestone quarry is to the south of this LCA.

Tertiary-aged basalts comprise a crudely-bedded succession of lava flows, columnar jointed lava flows, ash-falls and red-weathered horizons (or boles) of the Antrim Lava Group.

Normal faults of differing orientations cross the area, juxtaposing all the above formations. NW-SE oriented faults dominate the area. Many of these structures have been activated in the last 10,000 years during landslip activity.

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 varied topography of this LCA reflects an upland landscape of drift free bedrock, blanketed in part by a cover of peat. Only on the western boundary of the LCA are there very small areas of Late Midlandian till that were associated with the large ice mass that was centred on the Lough Neagh Basin and which moved approximately northeastwards across the region. 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 uplands such as the Antrim Plateau as ice free areas surrounded by encircling ice.

Key Elements

ASSIs

143 BLACK BURN (Mostly in this LCA, but overlaps into LCA 123)

Black Burn Cave is the only known extensive, active cave system developed in the Cretaceous age Ulster White Limestones in Northern Ireland and is probably the most extensive karst drainage system that includes an open passage developed within Cretaceous limestone in the British Isles.

067 GARRON PLATEAU

Interest lies in its geology and peatland flora and fauna. Geological interest centres around a successional sequence of olivine basalts and the presence of a picrite-dolerite plug formation at Trosk, which is unique in Ireland. This is the largest area of intact blanket bog in NI.

Karst Features

Nappan Turlough (split between this LCA and LCA 123)

This small lake, which fills and empties in response to ground water levels, is underlain by the Ulster White Limestone Formation. It has no visible inlet or outletand is commonly dry in summer.

AONB

All of this LCA lies within the Antrim Coast and Glens AONB (1988). This designation is indicative of the scenic quality of the landscape.