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Magilligan ComplexLondonderry
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Summary Full report
Site Type: Coastal section
Site Status: ASSI
District: Limavady Borough Council
Grid Reference: C665387
Rocks
Rock Age: Quaternary (Holocene)
Rock Type: Sand
Interest
Other interest: No data, Beach deposit, beach ridge, sand dune

Summary of site:

Magilligan, with an area of 32 sq km, is the largest accumulation of coastal sands and other associated deposits in Ireland. In the last quarter of the 20th century over 50 scientific papers describing aspects of the area have been published making it the most intensively studied of all Irish coastal deposits of the Holocene (the last 10,000 years).

The triangular expanse of Magilligan is made up of over 150 beach ridges (and possibly many more) that face north east. The first formed on a gravel ridge just north of the mouth of River Roe and the rest accumulated onto the advancing front eventually reaching the current position on the Atlantic coast. The ridge heights decline south to north from 7 m above modern sea level to around 2. 3 sets of ridges have been recognised. The first two, which are older, were formed by turbulent waves breaking on the shore (swash) carrying in sand which was deposited on the beach. The sand is largely quartz and pulverised shell debris with the odd large bivalve mollusc shell incorporated. Internally the ridges are composed of thin, parallel beds inclined by 1 or 2 degrees to the sea. The third set is made up of tapering ridges at Magilligan Point, recent deposits on the most distant reach of drift along the coast. The raised beach plain formed by the ridges has more recent deposits on its surface, particularly peat and shelly ooze (shell marl) between the ridges and wind blown sand covering the edges of the plain extending to a thinner, more general cover of up to a metre inland. These surface deposits render the plain almost flat and only along the north coast are there dune ridges and mounds, all of recent origin. On the Lough Foyle (western) coastline, sand reaches a maximum thickness of 5 m but on the north east, Atlantic, coast (Magilligan Strand) wind-blown sand forms a belt up to a kilometre wide reaching heights of around 17 m above modern sea level in places. These dunes contain large shell accumulations, some the result of prehistoric human activity. At present, the entire shoreline, with the exception of the distal tip at Magilligan Point, is undergoing severe to very severe erosion. The erosion cliff along the Lough Foyle shore offers clear evidence of the origin and development of the plain. It also clearly exposes the peat and freshwater shell marls in the inter-ridge depressions with leached, free-draining (podzolic) soils on the crests and slopes. Following the final retreat of ice at the close of the last glaciation, sea level began to rise in the period after about 9,000 years ago, reaching its maximum height above present sea level, of up to 4 m, between 7,000 and 6,500 years ago. This marine transgression flooded the lower Foyle and its estuary and remnants of an elevated cliff dating from this time can be traced around the lough. On the east side it was cut into boulder clay and the eroded material was transported south by the sea to form the gravel ridges, known locally as the Giant's Walk, just north of the present mouth of the River Roe. It was from this gravel bank that beach ridges started to develop as sea level began to fall (probably due to a rising land surface, adjusting to the loss of its loading ice). This rise also elevated the lough bed, reducing the volume of water flowing into and out of the lough in each tidal cycle so two important factors favoured the onshore migration and deposition of sand from the shelf just out to sea. The result was the train of ridges with its Atlantic front moving and expanding to the north east. At the same time, the ridges behind the front, probably initially tidal, began to stabilise and the depressions between started to fill with peat which has been dated to between 2,500 and 1,000 years before the present. The vegetation on top of the ridges created a soil based on free-draining sand. Inevitably this resulted in leaching of nutrients and the creation of a soil type called podzol. As the process of ridge addition neared completion, wind-blown sands crossing the ridges formed dunes near the coast, also with podzols that graded into fen peat and freshwater marls in the depressions, a landscape that was preserved by later events. Sea level stabilised at its current level between 2,000 and 1,500 years ago when the sand supply for ridge development was finally exhausted. From this time, erosion of the western, Lough Foyle, edge of the foreland commenced pushing the shoreline back progressively towards its modern position where it stabilised, protected by the intertidal sands that accumulated in front. This erosive phase also released sand that was carried east and north by waves driven by the prevailing winds, which also drove sand ashore, creating a mantle of northward migrating dunes, eventually eroding to create a flat plain of leached soil. The modern, discontinuous sand dunes lie on this surface. During the 17th and 18th centuries the Lough Foyle shore became the larges rabbit warren in Ireland which would have provided a further sand supply for the later dunes. Along the Atlantic front, the modern dune system rises to a maximum height of 17 m and is composed of blown sand generated by erosion of the beach. Radiocarbon dating indicates that this process began around 700 years ago. Within and beneath these dunes there are several shell mounds, some proved to be shell middens created by coastal dwellers in the early Christian period. From Benone to Downhill the coastline is stable and may even be advancing a little and at the extreme east end, the beach is backed by basalt cliffs partly masked by a ramp of frontal dunes. Here the tumbled and slipped basalt blocks falling onto the sand have created a complex landscape. At Benone where the road gives access to the shore concentrated leisure use has led to the loss of stabilising grasses resulting in large blow-out hollows in the dunes. Local overgrazing may also be a factor in vegetation loss. From this point westwards there has been rapid erosion, quantified since 1949 at around 3 m a year, a rate unmatched elsewhere in Northern Ireland. Some of the sand is lost offshore, some blown inland but the remainder is carried west along the shore to Magilligan Point. Immediately south west of the Point, on the Lough Foyle shore, erosion persists at just over a metre a year, the sand moving onto the banks in the mouth of the lough or migrating northwards to the Point. Magilligan Point itself is maintained by northward migrating bars in the lough, driven by wind generated waves and by swell from the north Atlantic which loses its ability to transport sediment as it approaches the point, creating an area of accumulation. The Tunns Bank, a sand shoal at the mouth of Lough Foyle, controls erosion and accumulation along the Magilligan shore where the beach and sub-tidal deposits seem to be linked into a self-adjusting system with a cyclicity of about 40 years. The bank appears to control wave refraction, growing as the shore erodes until a trigger point is reached and the process reverses. The sediments migrating towards the Point along the Atlantic and lough shores form a succession of beach ridges, eventually deflating into foredunes. This classic landform is of international importance and is comparable to any beach-ridge foreland in the British Isles or Europe. Its research base, precise chronology (based on 28 carbon 14 dates) and imposing setting, fully justify its Area of Special Scientific Interest (ASSI) designation and it is likely to be a baseline for future coastal research, particularly at a time of rising sea levels in response to global warming. It is also a superb and well-used educational resource.


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