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Weird Formation

pwhole

Well-known member
I photographed this strange thing a few years ago and it's always puzzled me what it is - does anyone have any ideas? Is it possibly a coral fossil?
 

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Looks like load structures, or flute casts formed by erosive scour. They usually occur on the underside of a bed, and are usually formed by turbidity currents (underwater sediment slides). They are most common on the underside of sandstone beds deposited rapidly as part of a turbidite flow, although they can sometimes be formed by tidal scour. Is it sandstone?
 
I was hoping you might know! No, it's in limestone - in Castleton, albeit not a bit regularly seen. It's in Pit Top Passage, high above the Bottomless Pit in Speedwell, which is mainly phreatic, developed on a fizzling-out vein - this is in one wall of the passage, and there's a tiny remnant of a vein just above it, so that may be a clue. It's the only occurrence up there (or anywhere that I've seen anyway), and seems to have been exposed by the miners. The colours are wonderful.
 
It's definitely as blue coloured as it is in the photo, and is definitely not galena - there's virtually none up there in that spot. Only the 'crust' is coloured blue or yellow. It's more of a formation that a mineral deposit as such - either limestone itself or a calcite deposition. It's the funny shape that gets me - it just looks very organic. I did see some pipe mineral that looked very much like Blue John in the Pit Props series but it didn't look much like this.
 
I've seen blue crusts like that in a few other places Phil, my suspicion is that it's a hydrated iron phosphate.
 
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