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Causeway Coast and Rathlin Island Geodiversity Profile

Last updated: 23 November 2006

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, where it forms a low, inwards dipping basalt plateau that defines its northern, coastal boundary. Although it is lower in altitude, this area has much in common, both scenically and structurally, with the Antrim Plateau to the east. However, its individuality was acknowledged by its designation as a separate AONB. The Central Lowlands as a whole 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 Causeway Coast and Rathlin Island character area on the north coast is characterised by the stepped profile and rocky knolls of the Middle Basalt. It is a plateau landscape with a distinctive rugged coastline where vertical basalt, rough grass, scrub and lichen covered cliffs fall to the stormy seas of the north coast. The coast includes unique formations of basaltic columns, stacks and needles, most notably the Giant's Causeway. Small, disused quarries, basalt outcrops, and sea cliffs give the area a broken character. The land rises to 207m AOD at Lannimore Hill. The contrasts between the black basalt cliffs, chalk and white beaches are striking. The Causeway Coast is designated as an AONB reflecting its nationally important landscape status and pristine condition. It is world famous for its geology and spectacular, pristine coastal scenery of dramatic cliffs and sandy bays. This international status has been recognised by the granting of World Heritage Site status to the Giant's Causeway for its geological and landscape value, making it one of only three such landscape sites within the U.K. The wild, open character of the indented coast would benefit from minimal intervention or disturbance in order to conserve the dramatic undisturbed character and long coastal views. Rathlin Island is geologically similar to, and can be considered as an outlier of, the Causeway plateau lands. It is also a designated AONB landscape as part of the Antrim Coast and Glens AONB. On the basalt plateau the presence of derelict stone buildings gives the landscape a slightly degraded appearance, although small-scale quarries often expose formations of geological significance. Many of the key sites within this LCA have received individual designation and protection (see below).

The coastal zone of this section of the North Antrim has been characterised by Orford (in Whalley et al. 1985) as being dominated by high cliffs cut into Tertiary basalts and in some cases the underlying Ulster White Limestone (chalk). Multiple benches can be distinguished in the cliff profiles and reflect differential weathering and erosion of individual lava flows and of red and grey palaeosols that make up interbasaltic beds. The presence of volcanic vents, faults and dolerite dykes create a typically embayed coast. Especially where the chalk crops out at sea level, bays can develop that contain extensive sand beaches and limited sand dunes. These may be backed by screes or, as at White Park Bay, slumped debris. Nearshore rhythmic bars are often associated with these swell dominated bays. It is, important to remember that such coastal areas are dynamic landscapes. Thus, even the hard rock Causeway Coast is subject to continuous erosion. For example, the cliffs between The Giant's Causeway and Dunseverick demonstrate a wide range of active mass movements including rotational and planar landslides, rock falls and toppling failure of basalt columns. Elsewhere, cliff collapse has threatened cultural sites at both Kinbane and Dunluce Castles. Despite its variety of designations, the area is not immune to human impact and there is ongoing controversy concerning the possible long-term impacts on coastal morphology of sand and gravel extraction from the beach at White Park Bay. The coastline is described in further detail by Carter (1991), including human impact on the coast, and in Knight (2002).

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)

Tertiary Intrusive Units - about 55 million years old
Tertiary Upper Basalt Formation - about 50 million years old
Tertiary Interbasaltic Formation - about 50 million years old
Tertiary Lower Basalt Formation - about 55 million years old
Cretaceous Hibernian Greensand & Ulster White Limestone - about 100 million years old
Jurassic Waterloo Mudstone Formation - about 200 million years old

This LCA extends from the north coast town of Ballycastle (east) to Whiterocks (west) and contains the classic Causeway Coast geology and geomorphology (ASSI 202). The geology comprises a mix of Mesozoic sedimentary and Tertiary igneous rocks in faulted and unconformable contact. The Giant's Causeway ASSI (ESCR Site 63), Dunseverick Slip (ESCR 76), Port Moon (ESCR 79) and Maddygalla Dyke (ESCR 85) occur in this area.Cretaceous

Basal fossiliferous, phosphatic, conglomeratic greensands (0-1m thick) rest unconformably on Jurassic strata, best seen on the eastern cove of White Park Bay (ASSI 107). Above the greensand occur the limestones of the Ulster White Limestone Group, spectacularly exposed in this LCA at Whiterocks (ASSI 174).

Tertiary

Lower Basalt Formation and Upper Basalt Formation

These two formations are separated by red, palaeoweathered beds and columnar basalts of the Interbasaltic Formation (IBF) as at Craiigahullier Quarry (with laterites) ASSI 115. Tertiary igneous successions also occur on Rathlin Island (also in LCA57) where the Ruenascarrive, Maddygalla and Doon Point locations fall into ASSI 033.

Dolerite and other Intrusions

Carrickarade ASSI (116) comprises a section through a Tertiary volcanic vent and neck, now filled with ash and agglomerate.

Faulting

There are two main orientations of normal faults throughout the area: NW - SE (like the dykes) and east - west. It is the latter fault trend that has influenced the orientation of a coast in the area of Ballintoy - White Park Bay and again (intermittently) from Portballintrae to Whiterocks.

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 largely upland plateau character, with a mixture of drift-free rock and till deposited by ice moving northward from Lough Neagh. The direction of this ice flow is indicated by the presence of numerous streamlined rock ridges. However the areas was also overridden at a late stage by southward moving Scottish ice. This was associated with the formation to the south of the Armoy Moraine (see LCAs 54 and 56). During deglaciation along the north Antrim coast, local and probably short-lived lakes existed in most of the coastal embayments and north-facing valleys during the melting of the ice and many of these may have drained south over what is now high ground. For example, at White Park Bay meltwater may have drained south through a valley west of Lannimore Hill, in which drainage now flows north into the bay. Another example of a major spillway occurs inland from the Giant's Causeway and runs for a distance of approximately 4km between Lisnagunogue and Causeway Head. This channel now only carries a small stream but, for a short time at the end of the last ice age, must have carried an enormous volume of meltwater. The drift map also highlights the dune sands at Runkerry.

Key Elements

World Heritage Site

The entire Giant's Causeway World Heritage site is located within this LCA

ASSIs

033 RATHLIN ISLAND COAST

Rocky coastline with associated flora and fauna. Nationally important colonies of Guillimot, Razorbill and Kittiwake breed on cliffs/sea stacks. Uncommon and rare plant species on coastal strip.

116 CARRICKARADE

The island of Carrickarade and the adjoining mainland represent a section through a volcano which exploded during the Tertiary period 60 million years ago. Volcanic ash and rock debris fill the vent. Later, during a quieter period of volcanic activity, veins of dolerite were intruded into the rocks. The limestone cliffs contain one exceptional raised sea cave which has the only known example of speleotherm development within the Ulster white limestone.

107 WHITE PARK BAY

Geological interest relates to the high chalk cliffs which display several exposures, from Jurassic age material belonging to the Lower Lias, some 200 m.y.o. to the Cretaceous rocks including the White Limestones which are some 80 m. y. o. . The calcareous mudstones and limestones are rich in fossils. Sea-stacks and natural arches are also present. The position of White Park Bay has also resulted in one of the finest displays of rhythmic bedforms in Ireland and beyond.

202 GIANT'S CAUSEWAY AND DUNSEVERICK

The Giant's Causeway is the most renowned geological site in Northern Ireland and has a truly international reputation. It is particularly important for its role in early debates on the origin of igneous rocks and is famous for its columnar basalts and associated materials. In addition, the Causeway Coast has representative Lower, Interbasaltic and Upper Basalt Formations relating to the three main eruption series in the Atlantic Ocean between 53-65 million years ago. The area is also important for its Zeolite mineralogy.

175 RUNKERRY

A beach system of international importance demonstrating beach states from dissipative to reflective. A wide range of rhythmic morphological features are present including beach cusps, rip current channels, longshore rip feeder channels, giant cusps and migratory transverse and crescentic nearshore bars.

101 PORTBALLINTRAE

Cliff exposure of glaciomarine, nearshore and raised beach deposits reflecting an isostatically controlled, emergent shoreline.

174 WHITE ROCKS

White Rocks contains the type sections for the Portrush, Ballymagarry and Tanderagee Chalk Members of the Cretaceous Ulster White Limestone Formation. Evidence of Tertiary explosive vent activity with associated agglomerate infill is present. The cliffs themselves are the best example in Ireland of coastal landforms developed in the Ulster White Limestone and include cliff, shore platforms, caves, arches, and sea stacks..

115 CRAIGAHULLIAR

The features of geological interest in Craigahulliar are a result of the volcanic activity experienced by this region during the Tertiary period, some 60 million years ago. The site is particularly important for the diversity of basaltic and related rock type and include the lower of the Interbasaltic 'red beds' and the first flows of the Causeway Basalts. The former has associated lignite beds with preserved macrofossils, while the latter occurs in a number of textures representing diverse cooling environments both on land and in or close to water. A range of unusual minerals are present together with notable lava cooling features such as columnar basalts, chisel marks and blister surfaces.

Other sites/units identified in the Earth Science Conservation Review

76 Dunseverick Slip Tertiary.

79 Port Moon Tertiary.

85 Maddygalla Dyke Tertiary.

AONBs

Most of the Causeway Coast AONB (1989) is included within the LCA, the eastern boundary encompasses a small part of the Antrim Coast and Glens AONB (1988).

Others

Runkerry strand

Runkerry strand is approximately 1.2km of beach at the mouth of the Bush river, backed by mature stable sand dunes. The formation of the primary gravel storm beach was probably associated with a rise in relative sea level prior to ca 7 000 years B.P., whilst the dunes developed later in response to a slowly falling sea level. For further information see the section by Jackson in:

White Park Bay

In addition to its geological interest. White Park Bay illustrates a range of geomorphological features. The landward slopes of the bay are made up of numerous shallow retrogressive slips overlain by aeolian deposits to form a 'chaotic' landscape. The beach and nearshore area consists of a strongly dissipative domain throughout the year. Off shore, there is a double series of crescentic bars that are thought to be the product of standing edge wave activity within the bay.