PETROGRAPHY OF NORWEGIAN MILLSTONES: A TOOL FOR VISUAL IDENTIFICATION OF QUARRY SITES

Millstone quarry sites from different periods are found in many places in Norway. Some are large quarry sites that have produced hand querns and millstones through hundreds of years, some more than thousand years. Others are small sites, involving production through a narrow time period or for a narrow marked. Here we present detailed petrography for identifying eight major groups of quarry sites.

The preferred rock for millstone production in Norway has, from the Viking Period until the production ended in the early 20th Century, been mica schist containing 'knots' (porphyroblasts) of hard minerals. The overall interpretation of the reason for this selection is that the combination of soft matrix minerals and hard porphyroblasts proved favourable for grinding cereals. The uneven hardness of mineral grains caused a sufficient relief on the grinding surface for efficient grinding, and also made the millstones 'self-sharpening' during wearing.

The embracement of one particular rock type for millstone production has, however, made it difficult for researchers today to find out the most likely source for millstones found in archaeological excavations. The method presented here for identifying the petrographic fingerprint of each quarry is developed through multidisciplinary studies combining the fields of archaeology, geology and hand craft development. The studies were performed as part of the four year running Millstone Project.

From a geological perspective, the quarry areas differ, in belonging to different geological provinces reflecting the metamorphic development in various provinces of the Scandinavian Caledonides. The similarity between mica schist within each province is larger than between the provinces. Thus, the mica schist quarries belonging to one and the same region are difficult to distinguish by petrographic examination, but querns and millstone from individual regions can be separated by performing detail petrographic studies.