Above: the surface of a red sandstone boulder found in till at Penblewyn, near Narberth. It's a really fascinating one, because it suggests TWO glaciations. In the first, the boulder was transported, abraded and emplaced somewhere. Could it have come from a coarse fluvial deposit? Possible -- but there appear to be facets on the boulder which would suggest glacial transport and modification. Then a weathering crust developed, with the darker coloured mineral veneer. That suggests a non-glacial interval. Then the boulder was picked up by ice again, with the surface damage we can see in the photo -- crossing striations and the rough-edged gouges which appear to have been smashed into the boulder surface. Such features appear all over the world on glaciated rock surfaces -- but nobody seems too sure what to call them. Percussion fractures? That's not all that appropriate, because percussion implies a sharp and very powerful impact. The process here is really one of a hard rock tool being forced down -- slowly and inexorably -- onto or even into a rock surface which is susceptible to damage. Continuously applied pressure is the thing. Should we call the features "pressure gouges" ?? Advice gratefully received........
These features are ubiquitous. See below -- a variety of moulded and striated rock surfaces from around the world, with these rough indentations all over the place, and not particularly well aligned with the striations. They seem to be concentrated on natural fracture lines. Sometimes they are fresh -- very rough and jagged -- and elsewhere they may be smoothed off, suggesting modification by some more fluid medium.
3 comments:
nothing to add about percussion marks, but that is an amazingly colourful rock. We are such a geologocal rich corner of Wales
Crush injury?
I have been pondering on the word "percussion" and wondering whether it should be used. "Percussion" in its old sense meant repeated striking of one object with another. So it was used -- for example -- to describe the process of flint knapping. But the word has now been hijacked by the music industry, and it is used nowadays almost exclusively in association with musical instruments. Yes, bedrock and clast surfaces can be "struck" by tools under pressure beneath or within a glacier -- but the process is very slow indeed, and although the pressure can be strongly exerted and then reduced, and then increased again, probably it is too slow for those who understand the word as currently used! So let's abandon it and stick with "pressure gouging" instead.......
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