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Bloody Bridge River Valley RidgesDown
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Summary Full report
Site Type: Inland exposure
Site Status:
District:
Grid Reference: J383270, J363263
Rocks
Rock Age: Quaternary (Late Glacial)
Interest
Other interest: No data, No Data, corrie, cross-valley moraine, valley glacier

Summary of site:

Evidence of glacial activity at the very close of Irelandís final ice age (the Midlandian) can be found in the Bloody Bridge river valley. It takes the form of curved ridges across the valley with the convex sides facing downstream to the east. These ridges take two distinct and easily recognisable forms. There are large ridges, up to 70m wide and 4m high, often studded with a few large boulders; and, in contrast, smaller hummocky ridges with rounded tops and a more arcuate form. The smaller ridges cut through the larger set and are therefore later in date.

The large ridges contain a higher volume of material and include large boulders and a much greater variety of rock types. They also reach further up the valley sides and extend all the way up the valley to the col between Slieve Donard and Chimney Rock Mountain. They are, in fact, moraines formed when the major glacier that filled the valley started to retreat as the climate improved. During this period it stabilised periodically. At these times the rate of flow was matched by the speed of melt back. When this happened the rocks transported on, in and under the ice were released all along the line of the ice front and gathered in mounds banked against the ice. When the glacier next retreated further up the valley, they were left as the ridges that we now see, somewhat degraded by about 13,000 years of later weathering and erosion. The sheer bulk, fragment size and variety of rock types indicate that the glacier was very large and flowed over a considerable distance. All the evidence indicates that it crossed the col west of Crannoge, flowing east, and then retreated along the same course. The variety of rocks and the fact that it breached the col suggests that it was part of the main regional ice sheet covering the Mournes at the height of the Midlandian, and the moraines form a record of the retreat of one of the peripheral glaciers. The smaller, later, ridges are also moraines but they tell a different story. After the main retreat just described, there was a further, relatively short, period when glacial conditions were re-established, allowing the snowfields round the higher summits to accumulate. In time they thickened and merged to form new glaciers that flowed under gravity into the nearest valley. At Bloody Bridge the snowfield that fed the local glacier originated on the north-east face of Chimney Rock Mountain, where a corrie began to form. The glacier from this corrie eventually reached the valley and followed the line of the earlier one to the east until it too reached the coast. The smaller moraines mark the final retreat of this glacier up the valley where it turned south west into the corrie on Chimney Rock Mountain, there finally to melt away.


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