Abstract
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).
Highlights
Limestones are common construction geomaterials that served either in the form of rough stones or dimension stones (Degryse et al 2003; Fronteau et al 2010, Viscusi-Simonin & Jaccottey 2013)
This sedimentary formation was exploited for the production of Neolithic and Protohistoric saddle querns (Monchablon, in press), as well as for rotary querns (Audebert & Le Quellec 2014; Jodry et al 2017; Picavet et al 2011; Pommepuy 1999; Robert & Landreat 2005), corresponding to changes in the sedimentological context: the limestone banks proceed from categories A1 to A2 (Figure 3a), to B, before become E type, which served mainly as a building stone (Devos et al 2010; Fronteau et al 2010)
The other limestones widely serving for the manufacture of millstones in the centre of the Paris Basin are from the Upper Lutetian (Eocene) formations, notably cerithids limestones and Lymnaea limestones (Buchsenschutz et al 2017; Fronteau et al 2017; Pommepuy 1999)
Summary
Limestones are common construction geomaterials that served either in the form of rough stones or dimension stones (Degryse et al 2003; Fronteau et al 2010, Viscusi-Simonin & Jaccottey 2013). Like many other sedimentary rocks, notably sandstones (Hamon 2006; Hamon & Fronteau 2018; Reniere et al 2016), limestones are endowed with a variety of lithological, petrographical and petrophysical characteristics even, at times, in the same geological formation, deposit or quarry (Picavet et al 2018). These rocks can serve to carry out certain functions or, in contrast, reveal characteristics that render them incompatible with an efficient use for a given function. Limestones, serving as paving or as decorative elements, are sometimes used like marbles, rocks with which they share certain technical characteristics without possessing identical textural properties (Brilli et al 2011; Ferrini et al 2012)
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