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Portstewart Strand Sand DunesLondonderry
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Summary Full report
Site Type: Coastal section
Site Status: PASSI
District: Coleraine Borough Council
Grid Reference: C805365
Rocks
Rock Age: Quaternary (Holocene)
Interest
Other interest: No data, Beach deposit, Coastal processes, buried podzol, buried topography, radiocarbon dating, raised beach gravel, sand dune, ventifact, washover barrier

Summary of site:

The mouth of the River Bann has been diverted to the west by east to west longshore drift of beach deposits. This diversion has created a dune complex, backed by the river, around 3 km long, narrowing from 1 km deep at its eastern end (west of the golf course) to half a kilometre at Ballyaghran Point where the river breaches the dune barrier and enters the sea. Their total area is roughly 1.5 square kilometres and the highest dunes, which are at the eastern end, achieve heights of 30 m.

Many of the key exposures that provided the evidence for the evolution of the dune system are now lost as modern landscaping and housing development have consumed the original landforms but sufficient early research was documented to be confident of the modern interpretation. The dunes stand on raised beach gravels with a surface at 1.4 m above sea level in the west rising to 4.5 m in the east. These gravels can still be seen between sand dunes. They consist chiefly of basalt pebbles with some flint fragments. Remains of over 100 mollusc species were found in the gravels including species now extinct in Northern Ireland. Wind facetted stones (ventifacts) have been found on the upper surface. The historic literature describes two levels of human occupation preserved in the dunes marked by black bands of soil and carbonized material. An inter-dune depression at the eastern end of the complex has also uncovered broken shells which yielding a carbon date of around 1,000 years before the present day. A dune ridge runs parallel with the shore line built by interaction of waves and the drift of sediment from east to west along the coast. Behind the ridge is a further series of ridges almost at right angles to the coastline, their windward faces constantly stripped of sand that then accumulates on the vegetated lee slopes. There are many blow-outs in the dunes, some resulting from destabilisation caused along well-used access routes but generally the dunes are well colonised by dune grasses and sea blackthorn, the latter introduced in the 1930s to assist stabilisation. Structural work at the east end of the strand in 1989-90 uncovered a 2 m vertical section 1.4 m of which was a podzolised (severely leached) sandy soil with a further soil buried beneath. This lower soil was carbon dated at around 4,800 years old whereas the overlying podzol was no more than 550 years old. During the Holocene period (the last 10,000 years) sea level rose and achieved its maximum height between 6,000 and 5,500 years ago. It was during this period of elevated sea level that the raised beach gravels were deposited probably originating from the reworking of glacial gravels lying in deeper water offshore. This opinion is supported by the rich mollusc fauna which is a mixture of intertidal and deeper water species. As the sea retreated wind became the major agent of construction winnowing sand off the beach and depositing it on the surface of the raised gravel beach. The ventifacts found in the inter-dune depressions standing on the surface of the gravel beach date from this period. There were times in the accumulation of the dunes when conditions stabilised sufficiently for early human settlement and both Neolithic/Bronze Age and Iron Age artefacts have been found associated with separate blackened horizons. A long history of dune accumulation is confirmed by these occupation levels. The shells in the inter-dune depressions that were dated at around 1,000 years old were almost certainly washed over the back of the beach in severe storm conditions. Dune construction has been a discontinuous process at Portstewart and some areas have been stable for long periods allowing soils to generate and some to be subsequently leached of nutrients. The podzol at the excavation site provides clear evidence of this. The Portstewart dune system records coastal events from the mid-Holocene to the present time (the last 6,000 years) and is one of the few places where such events can be compared and dated. The occurrence of ventifacts is unique in Ireland and the soil horizons provide excellent research material for the study of soil development in a variety of dune settings. All these factors, combined with the estuarine setting, give this site a national and international importance deserving recognition and sensitive conservation.


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