The Bletterbach is a Mecca for geologists from all over the world. 20 million years of the earth’s history are perfectly revealed here. No other opening offers such a comprehensive and many-sided insight into the sedimentary sequence in the southern Alps, from the Upper Permian to the Middle Triassic.

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leaf through the earth’s history book

A vertical history of the earth In the Bletterbach canyon there are rocks which come from the important transitional period from the Palaeozoic to the earlier Mesozoic era. Especially the Permian sediments are famous throughout the world on account of their various reptile tracks.

The rocks in the Bletterbach can be divided up as follows, starting at the bottom and working up: Bozen quartz porphyry, Gröden sandstone, Bellerophon strata, Werfen strata, and sarl-dolomite. The oldest stratum is the Bozen quartz porphyry, formed around 260 million years ago. The youngest stratum is the sarl-dolomite, which is about 240 million years old.

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fire out of the heart of the earth

Bozen quartz porphyry Quartz porphyry is a mainly volcanic rock strata, which can reach a thickness of over 1500 metres in the Bozen area. In addition to the typical deposits from hot clouds of embers (Ignimbrite), one finds tufa laid down by flowing water and streams of mud, and even river and marine sediments with remains of plants. In the Bletterbach gorge the quartz porphyry forms steep terraces with high vertical walls.

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sandstone, siltstone, claystone - tracks of prehistoric reptiles

Gröden sandstone Towards the end of the Palaeozoic era, the quartz porphyry was deeply eroded in places by the hot dry climate. From this, and even older eroded rubble, Gröden sandstone was formed. The 160 m thick formation in the Bletterbach consists mainly of coarse to fine river sediment, which was deposited in wide coastal plains. A special feature is a calcarenite bank by the waterfall, the famous cephalopod bank, evidence of a short-term ingression by the sea. On the basis of the many reptile tracks, plant remains, and sediment structures found in the sandstone, it was possible to reconstruct accurately, and in detail, the landscape, and the flora and fauna which lived in it. In the 16th century a number of mining galleries were excavated in the lowest levels of the Gröden sandstone, in order to extract copper ore. The mining was not particularly successful, however.

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rich in gypsum - the mass extermination

Bellerophon strata Gradually the upper Gröden strata give way to the dark grey Bellerophon strata, rich in gypsum. These sediments, named after a sea snail found in them, reach a thickness of about 40 metres. In the lower part of the formation there is still clear evidence of the influence of the nearby dry land, but in the upper section there are sediments rich in gypsum, which were built up by a strong evaporation of sea water. The border between the Bellerophon strata and the Werfener strata above, coincides with the Permian-Triassic border, and thus the border between the Palaeozoic and Mesozoic eras. At this border there was an unimaginable mass extinction of the marine animal life throughout the world. About 80% of all the animal species living in the sea died out in a relatively short time, for reasons that are still not entirely clear.

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flowing seas.

Werfen strata The Werfen strata reach a thickness of about 400m in the Bletterbach. They are a colourful sequence of strata, which reflect the alternation between the advancing and retreating Tethys sea. The basis of the Werfen strata was formed out of Tesero Oolite consisting of small chalk pebbles. This sediment was laid down near the beach in warm seawater, rich in calcium carbonate. Towards the top, the Oolite gives way to chalk, marl and claystone. The sediment structures, such as ripple marks, and cracks from drying out, indicate a shallow area of deposit, that may have been dry at times. In places the Werfen strata are very rich in fossils. There are various types of mussels and snails, and also marine single cell creatures, such as foraminifer and algae. The upper limit of the formation is the Richthofen Conglomerate, which is only a few metres thick, and is evidence of a period of pushing up and removal.

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South Sea dreams

Sarl-dolomite
Above the Richthofen Conglomerate there is, finally, sarl-dolomite, which forms the brilliant white summit of the Weisshorn. Sarl-dolomite was formed in a tropical sea with shallow, clear, well-aerated water, by the chalk-binding action of chalk algae. In the rubble of the Bletterbach, the sarl-dolomite is easy to recognise by its pale porous appearance.

Christian Aspmair

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