Seeing the Woods

A blog by the Rachel Carson Center for Environment and Society


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Snapshot: Where Geology Meets Early Modern History

A Millstone Quarry in Upper Bavaria

By Katrin Kleemann

 

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Photographs by Katrin Kleemann, CC BY 4.0. 

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.

 


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Photo of the Week: Francis Ludlow

Photo: Francis Ludlow

Photo: Francis Ludlow

Photo: Francis Ludlow

Photo: Francis Ludlow

These images show a piece of ancient Irish oak wood, in which the ring-widths can be counted and measured for size. Bigger size equals better growing conditions, and this piece of wood happens to span one of the most famous episodes of extreme climate globally in the past two millennia, occurring from c.536-550 AD. There is an ongoing debate about whether the event was caused by a massive volcanic eruption and/or a comet loading the Earth’s atmosphere with particles that reflected incoming light and dramatically cooled the Earth’s surface. In the image, the year 532 is marked, in which the tree grew very well. But starting shortly afterwards (and especially from 536) you can see how the rings become narrower and narrower, and even become difficult to see. This reflects the environmental downturn that was in progress globally at this time, and which has been linked to famines and mortality in written sources from Ireland to China. One report in early medieval Irish chronicles for 538 notes a “failure of bread”. That this event was noted at all at this early period of Irish history, when written records are very scarce, suggests the seriousness of the conditions experienced at the time. This image also reveals the environmental background against which the great sixth-century plague of Justinian occurred.

We thank David Brown of Queen’s University Belfast for permission to photograph this oak sample.