Taken for the Circles challenge at DPC
These millstones were once produced by hand from the native gritstone rock. Each matching pair of stones fashioned by an individual stone mason. At some point they became uneconomical to produce here and hundreds of finished and semi-finished millstones lay around the Stanage Edge area.

Here's a snippet from a historical web site which proposes a different reason for the abandonment of the Millstone Quarries:

The main reasons for the decline of this industry is thought to be that when it became fashionable to eat white bread, Peak District stones were not suitable, because they turned the flour an unpalatable grey colour!