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.
The magic line is particularly distinct in the Pizol region, between the Ringelspitz and Tristelhorn, at Piz Dolf, Foostock, the Gross Schibä - Piz Sardona - Piz Segnas - Piz Atlas group, at the Tschingelhoren and the Fil de Cassons.
Nappes moved not only along the Glarus thrust, there are about a dozen other thrusts in the Tectonic Arena Sardona, for instance the Mürtschen thrust in the Fessis-Mürtschenalp-Murgtal region and the Säntis thrust exposed along the cliff faces of the Kerenzerberg.