Stone runs are a fascinating feature of the Falklands landscape, occuring in areas of hard quartzite rock. Charles Darwin, after visiting in HMS Beagle in 1833-34, wrote in his journal "In many parts of the islands the bottoms of the valleys are covered in an extraordinary manner by myriads of great loose angular fragments of the quartz rock, forming 'streams of stones'...They are not thrown together into irregular piles, but are spread out into level sheets or great streams."
During the last ice age the islands were repeatedly deep frozen and battered by icy winds. Only a few small glaciers formed on the lee side of the highest mountains, but intense weathering of exposed rock on hilltops by repeated freezing and thawing left behind residual tors surrounded by frost shattered rock debris.
The local term stone run is used both for the extensive boulder fields up to a kilometre wide and to remarkable stripes on the hillsides produced by linear, parallel alternations of boulders and vegetated ground. Scientifically these are periglacial blockfields and blockstreams.
Cycles of freezing and thawing first broke up the rocks and then heaved and sorted the resulting debris. As the seasonally thawing ground slowly crept downhill over the deeper layer of permafrost it flowed out onto the lower areas.
Excavation shows that the largest boulders form the top part of a stone run with the size of the blocks decreasing downwards. The uppermost boulders have been leached by rainwater to a pale grey colour and are usually covered by lichens. Lower down, where the boulders have been protected from the weather, they are invariably stained orange by iron oxides.
During the last ice age the islands were repeatedly deep frozen and battered by icy winds. Only a few small glaciers formed on the lee side of the highest mountains, but intense weathering of exposed rock on hilltops by repeated freezing and thawing left behind residual tors surrounded by frost shattered rock debris.
The local term stone run is used both for the extensive boulder fields up to a kilometre wide and to remarkable stripes on the hillsides produced by linear, parallel alternations of boulders and vegetated ground. Scientifically these are periglacial blockfields and blockstreams.
Cycles of freezing and thawing first broke up the rocks and then heaved and sorted the resulting debris. As the seasonally thawing ground slowly crept downhill over the deeper layer of permafrost it flowed out onto the lower areas.
Excavation shows that the largest boulders form the top part of a stone run with the size of the blocks decreasing downwards. The uppermost boulders have been leached by rainwater to a pale grey colour and are usually covered by lichens. Lower down, where the boulders have been protected from the weather, they are invariably stained orange by iron oxides.
No comments:
Post a Comment