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 Coleraine Farmland landscape character area extends along the north coast from Castlerock to Portrush and southwards along the River Bann valley as far as Milltown. A series of broad ridges and valleys are aligned north-south, including the valley of the Bann itself, but this pattern breaks down to the east of the Bann, where there is a broader basin which extends to the higher land of Carnsheen and Moyle Hill to the east. The River Bann flows within a relatively narrow, wooded valley that widens to an attractive estuary landscape at the north coast. The area is underlain by the basalts of the Antrim Plateau, and the underlying structure of the basalt plateau is revealed where it breaks away to form rocky, indented cliffs at the coast between Portstewart and Portrush, with long coastal views to the Causeway Coast and to the Bann Estuary. The dark brown cliffs are weathering and eroding to leave rocky peninsulas jutting out into the sea. The ridges are open and quite windswept, but there are also more secluded 'pockets' of enclosed farmland, such as the area around Bush Hill and the landscape immediately to the south of Portrush. Some of the ridgetops are capped by rock outcrops.
The coastal area of this LCA has been described by Orford (in Whalley et al. 1985) as being created by a N-S structural depression in the Antrim lavas causede by two faults that displace the basalt in the trough some 350m below current sea level. The trough has in turn been filled with Tertiary clays and Pleistocene drift deposits. The River Bann drains Lough Neagh along this trough, and there has been considerable deposition along and across the mouth of the Bann as evidenced by the by the striking sand spit of Portstewart Strand and by ridges of rugged sand dunes with a distinctive, wild character. The coastline is described in further detail by Carter (1991), including human impact on the coast related in particular to sand and gravel removal, and in Knight (2002).
Pre-Quaternary (Solid) GeologyThe 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 Lough Neagh Group - about 20 million years old |
| Tertiary Intrusive Units - between 50 and 60 million years old |
| Tertiary Upper Basalt Formation - between 50 and 60 million years old |
| Tertiary Interbasaltic Formation - between 50 and 60 million years old |
| Tertiary Lower Basalt Formation - between 50 and 60 million years old |
| Cretaceous Hibernian Greensand & Ulster White Limestone - about 100 million years old |
| Jurassic Waterloo Mudstone Formation - about 205 million years old |
The LCA comprises 85% Tertiary basalts, the remainder being underlying Cretaceous and Jurassic (on the coast) and Tertiary Lough Neagh Group (part of the Ballymoney Basin in the southwest).
Jurassic
The oldest rock unit of the area comprises the Jurassic Waterloo Mudstone Formation, cropping out in the north of the LCA around Portrush. These marine mudstones are fossiliferous representatives of the Lower Jurassic or Lias. The shales are famous for being exposed on the coast at Portrush where ammonites are found in hornfels, one of the key observations of the plutonist vs neptunist debates of the 18th century: ASSI113.
Cretaceous
The Cretaceous succession comprises conglomeratic greensands (0-1m thick) unconformably on Jurassic strata. Above the greensand occur the limestones of the Ulster White Limestone Group. The type sections of the Portrush, Ballymagaree and Tanderagee Chalk Members occur at the White Rocks ASSI (174).
The mixed northern succession is intruded by sills (see below) and is brought into juxtaposition with the Upper Basalt Formation by the major NW-SE Portrush Fault.
Tertiary
Lower Basalt Formation and Upper Basalt Formation
The two Tertiary-aged basalt formations comprise a crudely-bedded succession of lava flows, columnar jointed lava flows, ash-falls and red-weathered horizons (or boles). These two formations are separated by red, palaeoweathered beds and columnar basalts of the Interbasaltic Formation (IBF), the stratigraphy is summarised below. Ramore Head ASSI (113) comprises the Ramore Sill which, like the Portrush Sill, has metamorphosed Jurassic shales (Waterloo Mudstone Formation) to hornfels. Lower Basalt Formation is resitrcited to the area south of Portrush and a second area in the far south of LCA54.
Dolerite Intrusion
The Portrush peninsula is formed by a major sill, a dolerite intrusion that is thought to extend offshore to form the Skerries. This sill is also associated with the igneous activity of the early Tertiary and is responsible for the local contact metamorphism of shales to hornfelses.
Quaternary (Drift) GeologyNorthern 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 map for the LCA demonstrates the complexity of an ice marginal zone that was first overridden by ice moving northwards along the Bann Valley at the time of the Drumlin Readvance. A late advance of Scottish ice then resulted in the Armoy moraine. The Armoy Moraine comprises a dominant feature of the local landscape that marks the southern extent of an advance of Scottish ice, which pushed sediments that had previously been deposited by an ice mass centred on Lough Neagh. The Armoy moraine is up to 4km wide, extends about 20km, northeastwards from Ballymoney into the Tow River valley and consists of large continuous series of ridges. Northwards and westwards from Ballymoney the moraine runs through the Seacon Ridge and, on the west of the Bann, as a sinuous deposit ending just north of Articlave. Sediments within the moraine are typical of lake deposits, probably laid down by meltwater emanating from an ice body centred on Lough Neagh. Exposures have shown that these deposits were then pushed from the northwest by an ice body of Scottish provenance. Exposures have also shown that the top of the feature has been planed off. Together with the distribution of streamlined landforms, rock ridges and drumlins, indicates that retreat of Scottish ice was followed by a northward advance of Lough Neagh-based ice. In the south of the LCA is a limited area of the Kilrea deglacial complex of outwash and deltaic sand and gravels. These are associated with the final withdrawal southwards of the Lough Neagh ice and most of the complex can be found in LCA 53. The drift map also highlights the presence of extensive coastal dune systems, the alluvial deposits associated with the Bann and post-glacial peat deposits in the southern half of the LCA.
Key Elements ASSIs174 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.
113 RAMORE HEAD AND THE SKERRIES
An intrusion of Tertiary dolerite has pushed into shales of Jurassic (Lower Lias) age, producing a fine grained, dark rock known as hornfels, which contains abundant fossils, particularly ammonities. This is the site of an historic geological argument about the origin of basaltic rocks. Evidence from this site supported the theory that basalt is formed from cooled volcanic lava. In addition, Portrush sill is an unusually complex layered intrusion.
208 BANN ESTUARY
Bann Estuary incorporates a series of sand dune systems that together with beaches and the lowest section of the River Bann, are part of the same physiological unit that has evolved over the last 6,000 years. The study of the dune sediments provides information critical to understanding sea-level history in the area and the development stages and processes in the evolution of temperate dune soils. Wilson and McGourty (2002) have described three small but distinctive dune systems that occur adjacent to the estuary of the river Bann. The Castlerock and Portstewart systems are fronted by broad, north-facing sand beaches exposed to the Atlantic, while the Grangemore dunes are within the estuary.
AONBsThe northwest of the LCA touches on the North Derry AONB (1966), whereas the northest of the LCA includes part of the Causeway Coast AONB (1989).
OthersPortrush - Portstewart SHORE PLATFORMS
Along the coast between Portrush and Portstewart are a series of exposed basalt shore platforms. Despite local vcariations, the platforms fall into two altitudinal categories, an extensive outer group just above mean sea level and another far more fragmentary inner group at about 2 - 3m O.D.. The higher group has been interpreted as relating to a higher late- or post-glacial sea level,, or even to an earlier, unspecified interstadial or interglacial sea level. However more recent research has indicated that the inner group may be related to modern surge levels, when wayer depth over the outer platform is sufficient to allow waves to pass over or to reform. Taken from: Whalley, et al. (1985).
Portrush dunes
The dune complex at Portrush is similar to many along the north coast of Northern Ireland. It was formed around 4 000 - 5 000 years ago as sea level fell away from its Holocene high that was locally about 3.0m above the present level. As the sea fell, the offshore islands of the Skerries exerted an influence on wave refraction and there was a switch from a concave to a convex platform. The dunes were a site of settlement from the Neolithic onwards, although their principle use now is for recreation. Taken from: Whalley, et al. (1985).