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Ballykelly Moraine ComplexLondonderry
No picture
Summary Full report
Site Type: Inland exposure
Site Status:
District:
Grid Reference: C267423, C26584183, C26274223
Rocks
Rock Age: Quaternary
Rock Type: Gravel, Sand
Interest
Other interest: No data, Deglacial sediments, Surge event, deltas

Summary of site:

The main events recorded in the glacial landscapes around Ballykelly and Limavady, and their sequence, are described in the Ballykelly site record.

This account adds more detail to the description of the deposits behind, and to the north, of the main arcuate moraine that extends from Clagan south to Drumraighland then sweeps east to Moys. Enclosed immediately behind this arc there is a series of ridges up to 30m high. They merge to the north west into pronounced rounded ridges extending as far as Ballykelly Forest. The ridges consist of glacial debris ranging from clay to pebbles arranged in beds near the back of the moraine, but at Moys they comprise cobble gravels (in a range of grain sizes) made up mainly of schist but with a small percentage of granite probably carried within the major ice sheet from Scotland. There is clear evidence that these deposits were disturbed by later ice movement. The second complex south of the A2 Ballykelly to Limavady road trends east-west and again individual ridges achieve heights of 30m. One good temporary break in the vegetation shows them to consist of pebbly gravels pitching 15 to the south and forming part of a linear deltaic front. Here the sediments are again mainly locally derived chalk, flint and basalt, but they also include a small percentage of Scottish granite pebbles. The moraine complex in this area records the dying stages of the last glaciation and the final surge of the Scottish ice into the Lough Foyle lowlands. Before it began to melt and decay northwards, this ice lobe blocked the Roe Valley and held back glacial meltwater lakes against the north-facing foothills of the Sperrin Mountains. The pattern of morainic ridges behind the moraine lobe demonstrate two different origins: at Moys the ice pushed or impressed the loose sediments into ridge forms whereas the northerly ridges between Ballykelly and Limavady were deltaic. This pattern matches well-researched deposits in the Barent Sea. The main morainic arc was formed by a submarine ice sheet surge with ridges behind reflecting the shape of the underside of the ice. Meltwater pulsing beneath the ice to the north formed deltas that were disturbed by later ice movement. The morainic ridges are a defining feature of relatively sudden movements of ice margins known as ice surges. The Limavady moraines preserve evidence of a large scale surge, unique in Ireland, and assist in clarifying the sequence of events as the ice retreated across the entire region. The only comparable features elsewhere in the British Isles are in east Yorkshire. These deposits are of national importance and selective intervention may be needed to clean up some of the key sections where dumping and inappropriate use threaten the limited exposures.


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