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Murrins Complex - Glensawisk Burn DeltaTyrone
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Summary Full report
Site Type: Inland exposure
Site Status:
Grid Reference: H532786, H557833
Rock Age: Quaternary
Rock Type: Gravel, Sand
Other interest: No data, No Data, deltas

Summary of site:

Glensawisk Burn is fed by two streams on the eastern slopes of Crocknakeeferty and from Stradowan Bridge flows north east for 2.5 km where it swings east for a kilometre then reverts to its north eastern trend for its final kilometre before entering the Owenreagh River in its upper course. This valley is occupied by a variety of glacial deposits but all appear to derive from a delta with its head in the vicinity of Stradowan Bridge. Just north of the bridge on the west side of the valley 3 abandoned sand pits have merged to present a section into these deposits 300 m long.

They occur in units, mostly of gravels, some without matrix, separated by faintly bedded thick sands of even grain size. Each of the units is defined by a contact that can vary from barely discernible to strong. Gravels sometimes grade into pebble beds with a matrix of gritty or coarse sand. One channel about a metre deep by 3 m wide can be seen, filled with three stacked units of granule to pebble-sized beds separated by faint sandy layers. Pebbles overlap in the bed of the channel which has a north north east/west south west orientation. These sediments were deposited into standing water, a glacial lake in the upper Owenreagh valley with its surface at 235 m above modern sea level. Meltwater from an ice front, retreating southwards past Mulderg, swept the gravels and sands into the lake where they flowed in plumes under gravity into deep water. Eventually the accumulating mass of sediment reached lake level at the point of discharge and a flat surface was formed which was then channelled by the force of surface streams crossing to the shore. Flow rates were not constant. Powerful flood flow, probably at the height of summer melt, was capable of carrying pebbles and gravel but weaker flow could only transport sands, graded according to the level of energy. The stacked and overlapping sediments are typical of deltaic accumulations. The slopes of the delta expanded into wider spreads of sediment on the lake bed, the sediments that now fill the valley of the Glensawisk Burn between the 180 to 220 m contours. The Glensawisk Burn delta is one of a series in the region believed to relate to the same phase of glacial retreat.

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