Limestone millstones: Facies, provenance and use of sandy to pure limestones in France

摘要

Limestones are sedimentary rocks more commonly associated with building stones or sculptures than with millstones. Nevertheless, many examples of limestone saddle querns, rotary querns and millstones are spread across France, at times making up the bulk of the archaeological assemblages in various areas characterized by bedrocks rich of sedimentary stones. These limestone millstones are of different types, sources and geological origins: Eocene sandy or fossiliferous limestones, mainly from the various limestones layers from the Lutetian beds, Quaternary calcareous tufas, and fine Jurassic limestones. To explain the behaviour of these rocks, this study advances a classification of the rocks used for millstones, focused not only on the rock-type but mainly on the topological aspect of the stone surface. (empirical macroscopic surface roughness) This renders it possible to classify the rocks into categories by materials, rather than according to petrographic facies. The pure limestones in this study are essentially vacuolar, whereas sandy limestones or certain biodetritital limestones belong to either the granular rocks category, which also includes sandstones, or to the category of heterogeneous rocks. These limestones appear for the most part to have been used due to their availability. Moreover, they epitomise a very satisfactory compromise between their grinding properties and their ease of carving, even if the hardness of these limestones is lower than those of other rocks used as grindstone (basalts or sandstones).