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Carey ValleyAntrim
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Summary Full report
Site Type: Inland exposure, Pit, River bank
Site Status:
Grid Reference: D30074361, D31274408
Rock Age: Quaternary
Rock Type: Gravel
Other interest: No data, Deltaic sediments, Glacio-fluvial sediment, Glaciolacustrine deposit, prograded delta, deglacial

Summary of site:

At the end of the Midlandian (the last glaciation in Ireland), the ice sheet that had covered the entire region began to melt and recede towards the higher land to the south. In the Carey River area it stabilised at Tormore where the rate of ice flow and the rate of melt were in balance, thereby keeping the line of the ice front stable long enough to form a small moraine (a bank of glacially deposited material). With thicker ice to the south, meltwater from under the ice flowed powerfully northwards from Loughareema (the Vanishing Lough). Large volumes of pulverized rock, ground by the passage of the ice sheet, were transported in this flow. The exposed land surface was also smothered in glacial debris which, in the Arctic climate, flowed in an icy mush downslope before it too joined the gushing meltwater.

As most of the coarse material was dumped in the lower valley and the flow of water eased slightly, finer sediments (a series of clays, sands and gravels) followed, to be swept into a relatively still body of water that had flooded the lower valley to a depth of 100m above modern sea level. As the feeding torrent entered this standing water it slowed, causing the sediment load to settle, spread out and form a flat-topped delta (as determined by the water level) with a steep front pitching into deeper water. New material was carried across the flat top, further extending it and spilling down the steep slope. This process continued for some time and built an extensive delta at what is now Drumadoon and near Carey House, it is still in almost pristine condition, though now high and dry. This delta ideally expresses the classic form called a Gilbert Delta normally developed where there is little wave or tide action. Water level then appears to have fallen suddenly and in the lower Carey Valley below the delta there appears to have been a descending flight of smaller terraces and deltas, reflecting a stepped series of reductions in water level. They were extensively eroded by the still copious flow of meltwater and now survive as mere fragments in the area north of Drumadoon. This later meltwater had cleanly sliced through the main delta along the line of the Carey River and provided sediment for the lower deltas and terraces. The above account presumes that the water body at 100m above present sea level was the sea. The weight of the original ice sheet is thought to have caused the land surface to settle into the deep, plastic rocks of the Earth’s mantle by 100m or more. As the ice sheet retreated southwards, the sea was able to flood in before the progressive adjustment of the land to its newly unloaded state could properly begin. There are other Northern Ireland examples of post-glacial features indicating similar sea levels. There is no direct evidence (e.g. fossils) to prove that the main delta was a marine feature and the lack of damage due to waves and tides is noteworthy, although the delta would have been protected by the enclosing higher land. Nor are there any temporary shore levels cut into the face of the delta, suggesting that the fall in sea level was sudden and dramatic. An alternative explanation might be that the delta spread into an ice-dammed lake that drained rapidly as the ice barrier melted, or perhaps it was breached - but again there is no solid evidence that Scottish ice advanced on to the coast. The Carey Valley is of national importance for its magnificently preserved Gilbertian delta and because it records a higher sea level due to isostatic (response to heavy loading) compression beneath massive thicknesses of ice. As well as its geological and geomorphological interest, the valley contains many ancient monuments. There are no immediate threats to the valley but any area so richly endowed with sand and gravel deposits is bound to be attractive to the construction industry and it is remarkable that there has been so little extraction to date. The granting of licenses to extract sand and gravel from the delta would quickly destroy the scientific interest and rich aesthetic appeal of the area.

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