Casletto Bridge

Along the Val Grande stream, on the rocky outcrops and the large boulders carried by the water, you can observe the spectacular features of what is known as Ceneri gneiss (named after the Monte Ceneri Pass in Ticino, Switzerland).

This rock, the origin of which is still the subject of heated debate, is the result of the metamorphism of a sedimentary rock believed to date from the Cambrian period. The most plausible interpretation is that it is a sandy-conglomeratic deposit originating from an underwater landslide (‘turbidite mass flow’) along an active continental margin above a subduction zone. During the lithification of the sediment (‘diagenesis’), carbonate concretions formed as a result of the carbonate-rich interstitial water. Metamorphism, resulting from a Carboniferous orogeny (Hercynian or Variscan orogeny), did not obliterate the structures created by these events. A great abundance of pebbles of various types and heavily deformed calc-silicate lenses are visible.
The mineral assemblage consists of: quartz, plagioclase, biotite, muscovite ± garnet ± sillimanite. The calc-silicate nodules are divided into zones with the following sequence (from the edge to the centre): biotite, hornblende, diopside, Ca-rich garnet and, occasionally, calcite.