Travels of an Urban Geologist: Building Bavaria II

The pretty, Medieval town of Nördlingen lies, fittingly, on the ‘Romantic Road’ in the west-central Swabian Bavaria. Its red-roofed buildings are enclosed within a complete and circular circuit of town walls. This is best viewed, along with the surrounding, rolling green countryside from the 90 m high tower, ‘The Daniel’ which is built over the west entrance of the town’s main church.

St Georg’s Church sits in the centre of Nördlingen, was built in the second half of the 15th Century. The tower was finally completed in 1639 and, because of the splendid views, it is named after a text from the Book of Daniel; “Then the king made Daniel and […] made him ruler over all the land” (2.48). The grey, breccia used to build the church came from the nearby quarry at Altenbürg, and is generally known as Bavarian Trass. ‘Trass’ generally refers to volcanic derived rocks which form a pozzolanic additive when mixed with lime cement, producing an hydraulic set and in indeed the trass from Nördlingen was also used for this purpose.

IMG_2018 IMG_1914Above left, St Georg’s Nördlingen and Right, Altenburg Quarry

But this is no ordinary, volcanic-derived Trass, such as those quarried in the nearby Rhine Graben. The Bavarian Trass of Nördlingen was formed by a meteorite impact.

Nördlingen lies in the Ries impact crater, created 14.5 million years ago (in the middle Miocene) from the impact of a meteorite. The crater is 24 km in diameter, and the bolide hit a target of Mesozoic limestones, up to 800 m thick which had been deposited on top of Hercynian basement rocks, granitoids, gneisses and amphibolites. At impact, the bolide was vapourised, the extremely high pressures exerted caused melting in the target rocks and of course an explosion, which threw molten rock, molten bolide and rock fragments upwards and outwards, this mix fell back down to Earth into the crater and surrounding areas. Impact melt breccias form a specific and distinctive rock type known as suevite, named after Suevia, the Latin for Swabia. The Ries Suevite has a grey, glass-rich, tuff-like matrix supporting angular clasts of basement derived granite and gneiss, keuper clay, sandstone, Malm limestone and slugs of glass. The latter where incorporated into the breccia whilst still semi-molten and these distinctive textures are known as ’Flädle’.

ries-map

ries-sectionA map and cross section of the Ries Impact Crater after Osinski (2004), showing the distribution of suevites in black.

IMG_1983 IMG_1952Above left; suevite in a quarry exposure, with coasts of granite and black Flädle. Right Flädle are obvious in the ashlars used in the church walls.

Visitors to Bavaria may well have eaten the delicious Flädlessuppe, which contains noodle-like strips of pancake, the Flädle. The variety found in suevite are somewhat less digestable, and as glass they contain molten components of both the target rocks and the vaporised bolide.

IMG_2009 IMG_2010Above, suevite used as building stone in the Daniel, with clasts of gneiss, limestone and Flädle.

Bavarian Trass, or Ries suevite, is probably one of the world’s most unique building stones. Though many would like to claim so, few churches have been built with an extra-terrestrial contribution.

References and further reading …

Osinski, G. R., 2004, Impact melt rocks from the Ries structure, Germany: an origin as impact melt flows? Earth and Planetary Science Letters 226, 529– 543.

Ries Geopark: http://www.geopark-ries.de/index.php/en/welcome

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About Ruth Siddall

Geologist, geoarchaeologist and co-author of The Pigment Compendium. Follow @R_Siddall; Facebook|Cultural & Urban Geology
This entry was posted in Building, Building Stone, Geology, Germany, Impact, Meteorites, Ries and tagged , , , , , , . Bookmark the permalink.

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