Val Pogallo

The mule track running along the Pogallo Valley from Cicogna provides an almost complete cross-section through the Strona-Ceneri area, the sandstone unit of the Laghi Series.

In the first section, we can see the ‘Gneiss Minuti’, metamorphic rocks derived from fine-grained sandstones, which still retain their sedimentary layering. We then pass through the “Ceneri Gneiss”, metamorphic rocks derived from coarse-grained sandstones and conglomerates, and, near “Ponte di Calenesc”, the augengneiss.

Beyond this point, the white augengneiss walls disappear and the slopes take on a regular profile. Along the path, one can observe the milonites associated with the Pogallo Line. Milonites are the result of plastic deformation along a fault that formed at great depth and high temperature. They are dark rocks with an extremely fine grain size, in which small, more or less flattened white lenses are visible. These are porphyroclasts, i.e. mineral crystals that have continued to behave in a brittle-elastic manner within the plastic matrix.

This extraordinary outcrop (first described by Boriani, 1970) represents the tectonic contact between the Ivrea-Verbano Zone, consisting of rocks typical of the lower continental crust, to the north-west, and the Laghi Series, consisting of rocks typical of the upper continental crust, to the south-east. This tectonic discontinuity is known as the ‘Pogallo Line’. Its identification made it possible to understand that the two units were not continuous, but had been tectonically juxtaposed. Subsequent studies (Boriani & Sacchi, 1973) have shown that the original contact between the two units is another fault, known as the ‘CMB Line’ (from the initials of Cossato-Mergozzo-Brissago, where it was identified). The Pogallo Line is more recent and displaces the CMB by a left-lateral slip of approximately 12 km.