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Mourne Plain Complex - Kilkeel CliffDown
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Summary Full report
Site Type: Coastal section
Site Status:
District:
Grid Reference: J32963126, J33163140
Rocks
Rock Age: Quaternary (Late Midlandian)
Interest
Other interest: No data, Glacial deposit, Glaciomarine deposit, Glaciotectonic, ice lobe, moraine ridge, outwash, raised beach

Summary of site:

Between the two endpoints indicated by the map references there is an inland cliff 14 m high and 2.6 km long. The base of the cliff is a diamict (a mixed sediment ranging in grain size from clay at one extreme to cobbles and gravel at the other) topped by stratified sands, gravels and cobble conglomerates from 1.5 to 4 m thick.

For 500 m from its western end the diamict is topped by cobbles and boulders with striated facets, clear evidence of their glacial origin, dragged along the bed of a body of moving ice. The diamict forms a strong contrast and sharp boundary with the overlying sands which are in strongly defined, cross-bedded units separated by plain sand and gravel beds. In other parts of the long outcrop, the sand is simply bedded with occasional hummocky cross-bedded units defined by both linear and undulating bedding planes. A few filled channels, up to a metre wide, can be seen in the sands. These in turn are overlain by sands and gravels, each bed of even grain size and many continuous for up to 100 m. The gravels coarsen upwards and the sequence ends with beds of cobbles with small amounts of finer sediments partly filling the space between them. The Kilkeel cliff exposes a section through a raised beach, one of four similar late glacial and post-glacial features. The sediments seen in the long sections are typical of the sandy shore zones from just below the fair weather base to the upper shore face where coarser material, particularly cobbles, tend to lodge. The zone of transition between offshore and onshore conditions is subject to rapid changes in water flow and the nature of the sediment transported. At the close of the final glaciation of the area the ice retreated from the coastal plain exposing it to marine erosion and deposition. In this area with immense amounts of glacial debris available on a low lying shore platform, deposition dominated creating the sediment sequences observed along the cliff face. But the departure of the ice, unloading the land, also caused it to rise relative to sea level as it attempted to reach an ice-free equilibrium. The shore deposits were eroded during this process of elevation forming the Kilkeel cliff which was itself finally lifted to a position beyond the influence of the sea.


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