Weird Formation

[pwhole]

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?

[Andy Farrant]

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]

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]

It looks a bit blue in the photo but if it is black and shiny I would suggest Galena or lead ore - PbS.

[moorebooks]

It looks a bit blue in the photo but if it is black and shiny I would suggest Galena or lead ore - PbS.

Could it be Blue John there?

Mike

[pwhole]

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.

[mikem]

? https://en.m.wikipedia.org/wiki/Stromatolite

[pwhole]

Ah! Now that looks closer, certainly. Thanks.

[AR]

I've seen blue crusts like that in a few other places Phil, my suspicion is that it's a hydrated iron phosphate.