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Thursday, 26 May 2011

More about Darwin's Boulders

 The two locations where erratic trains of granite are found, on the south shores of the Magellan Straits.  The source area of the erratics is shown in red on the top map.  Locations A and B are about 80 km apart, and there are no granite erratics in this intervening area.

I have blogged about these famous granite boulders before.  They are located in Tierra del Fuego, not far from the Straits of Magellan.  There are two erratic trains, made of boulders up to 16m long and apparently scattered across the landscape close to the coast.  They rest in a till sheet, but where they are found on the shore all of the other sediments have been washed away, leaving the boulders in glorious isolation, washed by the waves.
Evenson and colleagues have interpreted the erratic trains as resulting from localised and short-lived rockfalls or avalanches in the high mountains which were the collecting grounds for the glaciers that flowed northwards and north-eastwards towards the lowlands.  The erratics are very rough and angular -- which means they have been transorted on, rather than within, the glacier ice.  So the interpretation is very like that for the Foothills Erratic Train in Canada.

There are several noteworthy things here.  First, note that the two erratic trains are very narrow -- up to about 500m wide, with very few boulders outside the "train".  In case B, there are some anomalous boulders to the north of the main train, explained by the authors as being due to lateral spreading in the terminal ice zone of the glacier as it reached the coastal plain.  Both trains were laid down by the same glacier lobe, but during different glacial phases.  The other thing of considerable interest is the manner in which these two lobes are physically separate from one another, with a large belt of country between them which is quite free of these granite erratics.  This "empty zone" is about 80 km wide.

This study reinforces the idea that erratics are input (sometimes, not always) into glaciers during short-lived "pulses" which can then result in erratic trains of limited length being transported from source area to dumping area, with no tail or trail being left behind in the area traversed by the ice.

I keep on trying to explain this to the archaeologists, who simply refuse to listen.......

GSA Today
Article, pp. 4-10 | Volume 19 Issue 12 (December 2009)
Enigmatic boulder trains, supraglacial rock avalanches, and the origin of “Darwin's boulders,” Tierra del Fuego Edward B. Evenson et al

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