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Faughan and Dungiven Basins - The MurniesLondonderry
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Summary Full report
Site Type: Inland exposure
Site Status:
District:
Grid Reference: X26904095
Rocks
Rock Age: Quaternary
Rock Type: Gravel, Sand
Interest
Other interest: No data, Glacio-fluvial sediment, deltaic foresets

Summary of site:

Looking south west across the valley from the vicinity of Carn (on the main Londonderry road, on the western descent from Glenshane Pass), a step-topped, steep-fronted terrace with sand pits forms a prominent feature. This area, known as the Murnee Hills (or the Murnies), is a large deposit of glacial lake sediments; two flat-topped surfaces, at 230m and 245m above modern sea level, are obvious. The north and north-east slopes form the front of the feature and are steep with an even incline of around 35 from the horizontal. The deposit tapers backwards on to the northern flanks of the Tamniarin and Teeavan Hills. It consists of thick, horizontal beds of sands and gravels (derived entirely from quartzite) with internally inclined faces (foresets) tilted to the north east. The foresets appear to occupy broad channels. The horizontal layers above the foresets are the topsets (laid down by later streams crossing the delta top) and are composed of much coarser cobbles.

Ice wedge pseudomorphs have been described from here. They formed when frozen ground cracked and surface water drained in and froze, forming an ice wedge. When the ice eventually melted, months or years later, the ground was lightly compacted and cemented and held the shape, leaving a wedge-shaped mould that was then progressively filled by loose sand and gravel. The ice wedges could only form after the lake had drained and they indicate a later period of intense cold. The lower, flat-topped unit has been extensively quarried for construction materials and parts of the upper level have also been worked. Fragments of blanket bog that originally covered the entire surface still survive in places. The deposits were formed as a massive delta building outwards into an extensive glacial lake that occupied the valley during the glacial retreat. A wasting ice lobe, standing on the northern margin of the Sperrins, provided the meltwater with its load of debris; the main point of supply appears to have been the Altnasheskin meltwater channel. Initially the lake level stood at 243m above modern sea level but partial melting of its ice dam dropped the surface to the modern 230m contour. This led to the erosion of the newly exposed deposits and the creation of a second delta, with its surface at the lower level. It is believed that the valley was dammed by Scottish Ice somewhere in the upper Dungiven valley and not in the immediate vicinity of the Murnees.


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