Neolithic peoples are the first known to use stones such as this to grind seed and grain into flower. The lower, stationary, stone is a quern. A pitted grinding surface is a required quern characteristic and this one has that, I cannot see how a rounded, unstable bottom, is helpful. The upper, called a handstone, well polished from use.
Click any photograph for a larger view and use Ctrl-x to zoom in closer.
Wikipedia: “quern” and “metate.”
Humberstone and Santa Laura Saltpeter Works World Heritage page
Copyright 2020 Michael Stephen Wills All Rights Reserved