Sunday, September 6, 2020

Hutton On Erratics

 "There would then have been immense valleys of ice sliding down in all directions towards the lower country, and carrying large blocks of granite to a great distance, where they would be variously deposited, and many of them remain an object of admiration to after ages, conjecturing from whence, or how they came. Such are the great blocks of granite which now repose upon the hills of Saleve".

... James Hutton : Theory of the Earth (1795). 

This passage is quoted in Jamie Woodward's book The Ice Age: A Very Short Introduction, and it is one of the earliest attempts to explain 'erratic boulders'. These are boulders sitting on a surface made up of a different rock type than the boulder, indicating that the boulders have been transported from a far away terrain. The great blocks of granite observed by Hutton were scattered on a limestone landscape that was part of the Jura mountains on the border between France and Switzerland.  Just how these boulders got to their present location was the subject of a lively debate in the late 1700's and the 1800's. Hutton suggested they were brought there by glaciers. Other naturalists, taking inspiring from Scripture, proposed that they were transported by Noah's flood. 

Erratics implied that glaciers must have been much larger in the past. Hutton never developed his thinking about glaciers into a full explanation. That would take several more decades of debate. Erratics found in Alpine regions and on the European and North American temperate plains became part of a growing body of evidence for past climate change.

Closer to home, the picture below shows an 'erratic' made up of a high grade metamorphic rock and sourced from the mountains seen in the background. The location is Darma Valley, Kumaon Himalaya.

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