A clear, razor-sharp line slicing across the mountain faces catches the attentive observer's eye where the three Swiss cantons St. Gallen, Glarus and the Grisons meet.
This line is not man-made; it is a geological phenomenon called the Glarus thrust.  Above this "magic line" lie the darker rocks of the Verrucano group (250 – 300 million years [Ma] old). In the northern part of the Tectonic Arena Sardona brownish slatey flysch rocks (35 – 50 Ma) can be found below the line, whereas in the southern part light grey limestones (100 – 150 Ma) occur.

Through the collision between the ancient African and European continents beginning some 40 Ma ago, a kilometre-thick layer of rock was thrust over 40 kilometres northwards along the Glarus thrust! The thrust itself was at that time as much as 16 kilometres deep in the Earth's crust. The average northwards thrust velocity was a few centimetres per year, the thrust surface itself was in an environment of 320°C temperature and roughly 5 kilobar pressure.

Nowhere in the world can this phenomenon be observed and followed over such a long distance of 20 to 30 kilometres.