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John Rowley
I walk my dogs a lot around Mappleton on the holderness coast and pick up bits and bobs that look interesting. Today I picked up this boulder out of the glacial till and got to wondering how it formed. The limestone has been scoured and smoothed in the last glaciation but how does it then become encrusted in a ring of iron pyrite? Click image for larger version - Name: IMG_20180330_224212.jpg, Views: 34, Size: 3.44 MB
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prep01
The Iron Pyrites must have formed on the nodule during fossilisation and then glaciation scoured it off.
Colin Huller
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Unregistered
That was my dilemma. I have been taught that smoothed and rounded boulders are created by glacial action? That would mean the deposit of pyrites was post glaciation? Do smooth round limestone boulders occur naturally?

Thanks John
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Unregistered
Ah Colin I think I understand! Are you saying that this isnt an external "crust" of pyrites but seams of Pyrites running through what was once a much larger layer of the limestone? It would be interesting to section the boulder !

thanks John
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prep01
Hello John and I forgot to say, welcome to the forum. This kind of thing could be caused by erosion of a further outer layer and then flattening. They might not have travelled too far from their origin. Pyrite a a large subject, so go to Wiki for an overview and if you wish to you can explore further.
Colin Huller
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TqB
There are concretions fresh out of the Yorkshire Upper Lias that look just like that, complete with emergent pyrite crystals. 
Tarquin
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