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Wednesday 15 November 2023

Bullet-shaped clasts


I'm still rather intrigued by the Newall Boulder, also somewhat prosaically known as RSN18.  The top photo was taken by Tony H when we were in Salisbury Museum last year, and the other is from my study today, showing the boulder found in the garden the other day.

Regardless of how the boulders were shaped, and how they travelled from A to B, the similarities are striking -- bullet shape, weathering crust, clearly defined facets, rounded off or abraded edges, and what appear to be pressure fractures on what are otherwise abraded surfaces.  The bullet shape, with one pointed upglacier end and one blunt downglacier end where the rock surface is rougher, is typical of glacially transported clasts.

The Newall Boulder is a welded tuff referred to by some as a rhyolite; the Trefelin Boulder is an unspotted dolerite.  The Newall Boulder, relatively fine-grained, does have some features which I think are striations -- the other boulder has none.

The biggest difference between the two boulders relates to prehistoric and recent damage done by human beings.  The Trefelin Boulder is in pristine condition; but the Newall Boulder has had a hard time of it.  Both the tip and the back end of the clast are missing, and I agree with Kellaway that some of the damage has been done by our  prehistoric ancestors who were maybe thinking of making some stone tools.  The most recent damage (well documented by Bevins et al, 2023) has involved cutting out or knocking off at least five samples for geochemical and petrographic analysis, and the slicing of thin sections.  Vandalism, or necessary sampling in the cause of science?  I wonder whether it was necessary to destroy the boulder quite so comprehensively?


chris johnson said...

Yours seems to have two eyes and a mouth on a long head reminiscent of the dolichocephalic longs skull associated with the long barrow folks.

Or it might be Australopithecus anamensis, complete with crest and matching beard.

The museum really ought to do a swap. Yours belongs in the Stonehenge Museum.

Or might it be a trick of the light?

BRIAN JOHN said...

Hadn't noticed that! You clearly have heightened awareness, Chris, and a superior knowledge of aliens and cave men. Maybe I should put my boulder up for sale?