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

Travels of an Urban Geologist: Building Bavaria

IMG_3810Bavaria is a huge province of modern Germany. Recently I visited the southern parts, the Allgäu and Upper Bavaria regions, out on the Molasse Basin of the northern Alpine Foreland, staying in the town of Memmingen. The countryside looks like a picture-book, full of toy farms with green manicured grasslands spotted with white, plastered houses with red tile roofs (right). What is noticeable is there is less of the slate and stone seen in the French and Italian Alpine forelands. However in the few towns and villages I visited, churches and modern shop fronts featured stone masonry or cladding. The building stones used were striking in being predominantly fairly recent looking breccias and tufas. I just assumed they were from the Molasse. I pointed one out to my (non-geological) friends and Amira, a local, immediately said ‘my Dad will know exactly what this is’. Amira’s Dad did know, he told us it was ‘Biberstein’ and it was related to the Ice Ages. Dads always know these things. So I started to look into this a little bit more.

Now, I have spent many years studying and teaching Alpine Geology, but have pretty much managed to ignore the Ice Age geology of the region. Somewhere deep I recall the mantra of Günz, Mindel, Riss and Würm, the original glacial periods devised by Alpine geomorphologists and geologists at the turn of the 19th Century. During the glaciations which define the Pleistocene Stage of the Quaternary, the Alps were covered by a huge ice cap, with enormous glaciers descending towards the southern and northern forelands. In Upper Bavaria, large lobate piedmont glaciers coalesced to form the Inn-Chiemsee Glacier, which was at its greatest extent during the last glacial maximum around 21 thousand years ago (ka). This ice body excavated the famous moraine field around the town of Rosenheim and when it retreated, left behind the enormous glacial Lake Rosenheim, the remnants of which are the present day Simsee and Chiemsee. However this was the last of at least four major glaciations, the Würm. The Victorian glacial chronology has been considerably refined over the last century, but the terminology remains essentially the same. The four main glaciations, Günz, Mindel, Riss and Würm each lasted around 100 thousand years, separated by warm periods of similar lengths.

IMG_3820Glacial Lake Chiemsee, with the Bavarian Alps behind

Quaternary deposits from the Bavarian-Austrian Alpine foreland have been used as building materials since the Roman period. The most famous and most widely used are the Brannenburg Nagelfluh, the Hötting Breccia and the calcareous tufa deposits worked between the Inn Valley and Vorarlberg. A major advantage of these stones, compared with the bedrock of the Alpine series, is that they are soft and easy to quarry, hardening on surface exposure. Despite their young geological age, these Quaternary deposits have been surprisingly resilient to weathering and erosion. They often display large porosity, which, far from being detrimental, has contributed to the resistance to decay; the stones dry out more quickly rather than preserving water in small pore spaces. Having formed at the Earth’s surface and not having undergone major periods of burial or diagenesis, they are at ‘equilibrium with their environment’ (Mirwald et al., 2012). They have been use since at least the Roman period, and some still continue to be quarried today.

A locally sourced and much used stone is the wonderfully named Brannenburger Nagelfluh from Brannenburg am Inn, in the Inn Valley of Southern Bavaria, south of the town of Rosenheim. This is Amira’s father’s ‘Biberstein’ named from the Biber hill near the quarries, and Biberstein is the colloquial name for this stone. The word ‘Nagelfluh’ is used in the German geological literature to refer to young (Tertiary or Quaternary) formations of conglomerate. Nagel means nail, and the name refers to the fact that in outcrop, the rock surface appears to have large nails hammered into it, so that only the heads are seen. Brannenburger Nagelfluh is a (just) coast supported, polymict conglomerate with a ratio of clasts:matrix of 50:50. The clasts are moderately sorted, up to around 10 cm across and composed of a variety of rock types derived from the Alps; limestone, sandstone, gneiss, amphibolite schist, dolerite and quartz. Brannenburger Nagelfluh formed on the shores of the Rissian Lake Rosenheim, and they represent a series of deltaic deposits, which are exposed in the quarries around Brannenburg and Flintsbach (Herz et al., 2014). These sediments were deposited at around 150 ka. The topsets are exposed in the Anton Huber Quarry and foresets are exposed in the Grad Nagelfluhwerk quarry. Grey and yellow varieties are observed in photographs of the quarries published in Herz et al. (2014).

IMG_3586 IMG_3590 Above: Shop fronts in Memmingen

IMG_3929 IMG_3910Brannenburger Nagelfluh used in Ludwig II’s crazy fountains at Schloss Herrenchiemsee

A superficially similar stone is the Hötting Breccia. This is an alluvial fan and talus slope deposit, developed on the Northern Calcareous Alps basement. The breccias outcrop around Innsbruck, and are dated between 100-70 ka (Spötl & Mangini, 2006). They are therefore associated with the Riss-Würm interglacial period. Petrologically, they are carbonate-cemented breccias, with poorly-sorted clasts of the underlying Triassic limestones. Local concentrations of red Permian sandstones (Alpiner Buntsandstein) stain the lowest deposits of the Hötting Breccia yellow and red. These are up to 40 m in thickness. The overlying White Hötting Breccia does not contain Buntsandstein, and has only limestone clasts, however this was less well consolidated and was not used for building (Unterwurzacher et al., 2010). Several quarries operated near Innsbruck until the early 20th Century, the largest of which was Mayr (below). Transportation both north and south along the river Inn was favourable in the distribution of this stone and it is the main building stone in Innsbruck, where it was used for the Cathedral and other examples of civic architecture (Mirwald, 2012, Unterwurzacher et al., 2010).

MayrQuarry2

Many modern buildings are clad with a grey matrix-supported breccia. We need to travel to the southern Alpine foreland to source this stone. This is ‘Ceppo’ from the shores of Lago di Iseo in the Italian Lake District. The Ceppo di Poltragno Conglomerate is a grey, carbonate-cemented breccia. Quarries are located in the Adda and Brembo valleys (Vola et al., 2009). Ceppo is a Quaternary fluvial-glacial conglomerate. It was deposited during the lower to middle Pleistocene (1.8 – 0.125 Ma) as diamictites and colluvial scree deposits. The clasts are matrix supported and are derived from the Triassic dolomites in the Carnic Alps (Forcella et al., 2012).

IMG_3569 IMG_3570 Above: Cladding on the ground floor of an office building in Memmingen

Ceppo has been quarried since the Roman period and is actively quarried today, used as cladding and as paving. Vola et al. (2009) have described its use in Bergamo and Bugini & Folli (2008) have described its use in Milan. Varieties known as Ceppo Gentile, Ceppo Gré and Ceppo Poltragno are marketed. Vola et al., (2009) list the following quarries; Camerata Cornello, San Pellegrino and Brembate Sotto (inactive) and Poltragno, Pianico, Grè (at Solto Collina on the lake shore) and Castro (active). The stone is exported throughout southern and central Europe and is widely used in southern Germany.

Back in Austria and Bavaria, tufa limestone occurs commonly across the Northern Calcareous Alps, found in association with cool spring activity, where waters are supersaturated with calcium carbonate. Important quarry sites are at Thiersee, in the Inn Valley south of Brannenburg and at Andelsbuch in the Vorarlberg of westernmost Austria. It could be relatively easily exported to the Allgäu region of Bavaria (Kempten and Memmingen) from this latter locality, along the River Iller. The stone is both strong and light in weight. Like many tufas and also large porosity stones such as the Portland Roach and the Florida Coquina, these stones are extremely effective in the constructions of fortifications as their properties allow them to absorb impacts (of cannon balls etc.) well. Thiersee Tufa was used to construct the Kufstein Fort in the Tirol (Mirwald et al., 2012). However there is good evidence that these stones have been won since the Roman period; they are used in the villa at Rankweil in the Vorarlberg (see Unterwurzacher et al., 2010). It is probably Andelsbuch Tufa that is used in the church at Memmingen (below).

IMG_3613 IMG_3609

At Thiersee, the tufa deposits are up to 10 metres thick and extended laterally, prior to quarrying for several hundred metres. They are primarily ‘moss tufas’ (Unterwurzacher et al., 2010). These deposits are no longer active and the deposits are almost worked out; peak production was in the 18th Century. At Andelsbuch, moss tufa and phytoclastic tufa have formed on top of the Helvetic Nappe and Flysch formations. The deposit is up to 5 metres thick and extends for 100 m. Tufa deposition here is still active (Unterwurzacher et al., 2010).

References and further reading

Bugini, R. & Folli, L., 2008, Piedras de la arquitectura milanesa (Stones used in Milanese architecture)., Materiales de Construcción, 58, (289-290), 33-50.

Ceppo di Gré: http://www.naturalstoneinfo.com/download/bgcamcom.ceppo_gre.pdf

Forcella, F., Bigoni, C., Bini, A, Ferliga, C., Ronchi, A., Rossi, S. et al., 2012, Note Illustrative della Carta Geologica d’Italia 1:50 000; 078 Breno., Servizio Geologica d’Italia., 313 pp. http://www.cartografia.regione.lombardia.it/metadata/carg/doc/Breno_note_illustrative_dicembre_2012.pdf

Herz, M., Knipping, M. & Kroemer, E., 2014, The Rosenheim Basin: Würmian and Pre-Würmian deposits and the Höhenmoos interglacial (MIS 7). in: Kerschner, H., Krainer, K. & Spötl, C., From the foreland to the Central Alps: Field trips to selected sites of Quaternary research in the Tyrolean and Bavarian Alps., Excursion guide of the field trips of the DEUQUA Congress in Innsbruck, Austria, 24–29 September 2014., 6-17.

Huber Quarry: http://www.nagelfluh.de/INFO/GESCHICHTE/tabid/58/Default.aspx

Mirwald, P., Diekamp, A., Unterwurzacher, D. & Obojes, U., 2012, Weathering of sedimentary stone materials formed under Earth surface conditions., 11 pp.

Sanders, D. & Spötl, C., 2014, The Hötting Breccia – a Pleistocene key site near Innsbruck, Tyrol., in: Kerschner, H., Krainer, K. & Spötl, C., From the foreland to the Central Alps: Field trips to selected sites of Quaternary research in the Tyrolean and Bavarian Alps., Excursion guide of the field trips of the DEUQUA Congress in Innsbruck, Austria, 24–29 September 2014., 81-93.

Spötl, C., Mangini, A., 2006, U/Th age constraints on the absence of ice in the central Inn Valley (eastern Alp, Austria) during Marine Isotope Stages 5c to 5a. Quaternary Research, 66, 167-175.

Spötl, C., Starnberger, R. & Barrett, S., 2014, The Quaternary of Baumkirchen (central Inn Valley, Tyrol) and its surroundings., in: Kerschner, H., Krainer, K. & Spötl, C., From the foreland to the Central Alps: Field trips to selected sites of Quaternary research in the Tyrolean and Bavarian Alps., Excursion guide of the field trips of the DEUQUA Congress in Innsbruck, Austria, 24–29 September 2014., 68-80.

Unterwurzacher, D., Obojes, U., Hofer, R. & Mirwald, P., 2010, Petrophysical properties of selected Quaternary building stones in western Austria. In: Prikryl, R. & Török, A.; Natural Stone Resources for Monuments., Geological Society of London, Special Publication, 333, 143-152.

Vola, G., Fiora, L. & Alciati, L., 2009, Stones used in Bergamo architecture., Studia Universitatis Babe-Bolyai, Geologia, 2009, Special Issue, MAEGS – 16, 137-139.

©Ruth Siddall 2015