A Millstone Quarry in Upper Bavaria
By Katrin Kleemann
The Mühlsteinbruch Hinterhör in Altenbeuren, Upper Bavaria—this millstone quarry was the first stop on a recent LMU geology field trip to the Northern Limestone Alps. The site is an official geotope of Bavaria (geotope means “Earth place” and refers to a spot in nature where the Earth’s history becomes visible). At this unique spot you can learn about the area’s early modern history as well as its geology; this is one of very few geotopes that was formed by humans.
Between 1572 and 1860, the quarry was used to produce millstones for the surrounding mills (which were transported on the nearby river Inn), explaining the characteristic round shapes that can be seen at this outcrop. But without the perks of modern technology, how did the quarry workers extract the circular millstones? Using a chisel and hammer to create the outlines of the round shape, they would drive wooden wedges into these circular borders and water the wedges regularly causing the wood to expand and fracture the rock, freeing the millstone from the wall.
Yet it is not just these insights into the innovative techniques used by people in the past that are revealed; this spot is also particularly interesting from a geological perspective. The 28-meter-thick layer is mainly made up of sandstone and marlstone, and belongs to the Helvetic Nappes; sheet-like bodies of rock that were once located in the shallow waters of the southern margin of the European continental shelf, created before the Alps were formed. During the formation of the Alpine mountains, the Helvetic Nappes (layers) were thrust northward and upward, deformed by the continued folding during the Cretaceous and Paleogene periods, explaining why a once-seabed is now found more than 500 meters above sea level. Fossilized oysters with thick shells can be found there, indicating that this spot must have once been located either in shallow water or perhaps at a river delta. The oyster shells had to be thick survive the pressure of strong tidal currents in the coastal waters.