New York, June 24 (IANS): How did people in the 18th and 19th century US choose millstones to grind food grain? They chose it on the basis of size of cavities or dots on the surface.
One of the key features useful to millers was the porosity of the rock - where fossils left avities.
In 1795, famous US inventor and millwright Oliver Evans exclaimed that pores "larger in diameter than the length of a grain of wheat" were not desirable.
The Society for Sedimentary Geology (SEPM) has described an unusual method that combines forensic palaeontology and archaeology to identify origins of the millstones commonly used in the 1800s.
While all millstones were used similarly, millstones quarried in France were more highly valued than similar stones quarried in Ohio, US.
Over four years the scientific team located millstones by visiting historical localities in Ohio, then studied them and identified unique characteristics between the coveted French buhr and the locally sourced Ohio buhrstone.
Both types of millstones were composed out of an extremely hard rock called chert, and superficially they can look very similar.
With close examination, the scientists confirmed that the French buhr contained fossils that came from a freshwater environment, including algae and snail fossils, and the Ohio buhrstone was quarried from a much older rock unit.
The Ohio rock unit, likely laid down in a Paleozoic marine environment, was filled with invertebrate fossils known as fusulinids, pelmatazoans and brachiopods.
Based on the fossil assemblages, the authors suspect the assemblages from the Ohio buhrstone may have made these millstones less effective for milling, but that this claim would require further investigation.
The paper was published in the journal PALAIOS.