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BuckyE wrote:
Do any millstones have concentric circles? The few I've seen had tangentially radial grooves, to lead the flour out to the sides. It seems to me a set of concentric ridges would not mill very well. But I'm not the expert, there.
You're right there, I think. But then again ... that could be why someone smashed it up. An aborted attempt at something different. A spiral would send stuff in- or outwards depending on how you turned it.

Mechanical innovation is not unknown in Bolton - Vernon Kaye's Reciprocating Mouse Sprocket immediately comes to mind. Though the area is well known to local historians, through maps and deeds, there are no accounts of flour mills ever. Just the placename inference that there was a Saxon fulling mill on the riverbank (about a hundred yards from where tuesday grew up), which is a quarter mile downhill from where the fragment is now. When stones are being transported for walls and buildings, in the field, they are almost inevitably taken downhill. The invention of internal combustion changed that, of course.

perhaps you've found the original cosmic millstone that grinds out stars. or islands. or something.
http://www.indigogroup.co.uk/edge/cmill.htm