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Grangemore and Castlerock Sand DunesLondonderry
No picture
Summary Full report
Site Type: Coastal section, Inland exposure
Site Status: PASSI
District: Coleraine Borough Council
Grid Reference: C805355, C780361
Rocks
Rock Age: Quaternary (Holocene)
Interest
Fossil Groups: Foraminifera, Plant
Other interest: No data, Beach deposit, Coastal processes, Estuarine deposits, buried podzol, decalcified sands, estuarine clays, pollen analysis, radiocarbon dating, sand dune

Summary of site:

Coastal sand dunes form wherever there is a source of sand and a prevailing onshore wind. Dune formation is rarely a simple process and it is often difficult to establish the exact events that create a coastal dune complex such as this one at the mouth of the Bann.

On either side of the mouth of the River Bann there is a dune system, extending from Portstewart (3km to the east) to Castlerock (1km to the west). This account describes the dunes on the west and south side of the Bann, now isolated into two areas about 1.5km apart. The Castlerock dunes are on the coast and immediately behind on the west side of the estuary. The Grangemore dunes are on the south bank, not quite 2km upstream. At neither site do they achieve great height: at Castlerock the maximum elevation is around 20m while most of the Grangemore dunes are below the 10m contour. These unspectacular sites are of interest for the estuarine clay and soil horizons they contain, which record sea level and climate changes over the last 10,000 years or so. The Castlerock dunes are now of little scientific significance since their earlier interest has been largely destroyed in the creation of the golf course. The Grangemore dunes, situated east of the confluence of the Articlave River with the Bann, despite their drab pasture and heath surface, contain much of interest and are the only ones not to incorporate shell fragments. Early reports described a bluish clay, containing the microscopic shells of foraminifera, just above high water mark in the banks of the Articlave River. Another early account described alternating, fossiliferous sands and clays in a stream about 1.3km west of the Articlave, again slightly above the high water mark. In both cases the fossil fauna was estuarine in nature. The surface of the clays was covered with blown sand. In the early 1980s an interdune deposit 50cm thick was found in the Articlave River, at about modern sea level, full of leaves, fruits, mosses and pollen, sandwiched between sands. Radiocarbon dating of these fossils gave an age of just over 5,000 years. The sands above this bed contained the remains of two immature soils. Podsol soils (soils with surface layers leached of minerals and fine material) in fine sands were again found at two places in the dunes in the early 1980s and carbon dates indicated an age around 2,500 years. The estuarine clays in the Grangemore dunes suggest that a middle Holocene marine incursion (between 5,000 and 6,000 years ago) reached a little above modern sea level and there is no evidence to suggest that it rose any higher. This fits well with other evidence from the wider area, with dates nearer 6,000 years ago. The bed of plant remains probably represents the flooding of a dune slack by the Articlave River at that time. The sands below that horizon predate it and indicate a lower sea level before the marine incursion. If this interpretation is correct, it would make these the earliest known dunes anywhere in Ireland. The dated soils suggest that the dune surface stabilised to some degree before 2,500 years ago and similar stable surfaces are known from the Portstewart dunes. It would be expected that soils formed on dunes would be free-draining and would therefore leach quickly. The evidence of estuarine conditions in the middle Holocene period at, or above, modern sea level makes the Grangemore area particularly significant and the entire dune complex of Irish importance. There is a case to be made for the protection of surviving dune systems from destructive developments, such as golf courses, which seriously modify surfaces during landscaping and then effectively sterilise them. In the case of Castlerock dunes, it is already too late but the surviving interest is, fortunately, protected by current management practices.


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