Previous SiteNext Site

Bloody Bridge River FanDown
No picture
Summary Full report
Site Type: Inland exposure
Site Status:
District:
Grid Reference: J383270, J384270, J387269
Rocks
Rock Age: Quaternary (Late Midlandian)
Interest
Other interest: No data, No Data, glaciofluvial fan

Summary of site:

In the lower 800m of the Bloody Bridge River valley is a wedge of sediments expanding from a point up-river to a spread 260m wide where it enters the sea. Their composition and structure explain the final events of the westward glacial retreat up the valley.

There are three exposures in these deposits which are now being rapidly eroded by the modern river. Two are on the northern bank, 800m and 700m from the coast; the third, on the southern bank, is about 400m inland. The most westerly is an exposure of coarse cobble and boulder gravel deposited as a crude bed. The cobbles and boulders are almost entirely of Mourne granite, the largest reaching 1.4m in size. The second, downstream and also on the north bank, exposes the somewhat obscured contact of the deposits with the bedrock, here massive greywacke (a muddy sandstone) of Silurian age (about 435 million years old). Again the base of the sediment is a cobble and boulder bed, this time made up of the local greywacke. Pebbles and coarse sand fill the spaces between the boulders. Above this crudely defined basal bed the sediment becomes finer grained, mostly grit to pebble size, and is laid down in beds up to 10cm thick. There appears to be no interruption between them, suggesting continuous deposition. Gravel beds thin to nothing downstream. There is a 2m deep channel washed out of the gravel, its bed filled with cobbles and boulders, mostly of granite, with more gravel fill above. The final exposure, on the south bank, is only 1m deep and lies on an undulating eroded surface of diamict (a glacial paste of mixed grain sizes). There are a few boulders at the base, then thin beds of cross-bedded coarse sands and pebble beds take over, in turn pinched out, to be followed by cobble and boulder beds. Flat stones tend to overlap like fish scales (imbricate structures), each with its upturned face directed up the valley. These deposits overlay everything else in the valley and are therefore a record of the last events during the melting of the last glaciers in the area. The boulder gravels suggest very close proximity to the glacier snout and could represent the collapse of coarse moraine material into meltwater channels. The finer, well-bedded sands settled from fairly fast flowing streams after the spaces between the boulders had silted up. The channel with its boulder fill was caused by a much more forceful water flow, cutting through the earlier deposits. Generally the sandy beds become finer and thinner downstream as the meltwater lost its capacity to transport coarser material, but pulsing floods seemed capable of carrying cobbles even at a very late stage. All these deposits appear to have formed when meltwaters washed out material from a coarse bouldery moraine in the corrie on Chimney Rock Mountain (see site record ‘Bloody Bridge River Valley Ridges’) in the very final stages of glacial retreat around 13,000 years ago.


Previous SiteNext Site

|Home|