Weird Formation
- Thread starter pwhole
- Start date
Andy Farrant
Active member
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?
pwhole
Well-known member
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.
rhychydwr1
Active member
It looks a bit blue in the photo but if it is black and shiny I would suggest Galena or lead ore - PbS.
moorebooks
Active member
rhychydwr1 said:
Could it be Blue John there?
Mike
pwhole
Well-known member
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.
