Unobvious geoheritage in sacral buildings: millstones in churches of NE Poland from a geological and geomorphological perspective

Abstract

The article highlights the importance of medieval churches with embedded millstones as geocultural objects with a great geotouristic and educational potential. In the lowland areas of Northeastern Poland, 79 millstones and their semi-finished products were inventoried. Their sizes ranged from 70 to 100 cm in diameter. The majority of them were made of erratic boulders, mainly granitoids of Fennoscandian origin. Additionally, gneisses, sandstones, pegmatites, and basaltoid were also identified. An attempt was made to determine the degree of weathering of the millstones over several hundred years of exposure to external conditions. Measurements of moisture content and salinity of the walls surrounding 10 millstones embedded in the walls of 8 churches were taken three times (in spring, summer, and winter). Lower wall moisture was observed in the vicinity of the millstones, but only in the case of three walls, the salinity of the mortar binding the bricks and stones was at a low level. A weak but statistically significant negative correlation was found between the age of the churches and the hardness of the millstones measured with a Schmidt hammer.