Pian d’Arla–Ospedaletto

The outcrop showsan alternation between bands of amphibolite (a dark rock containing plagioclase and hornblende amphibole) and leptinite (a light-coloured rock containing plagioclase and quartz). These are probably original tuffites, that is, products of explosive volcanic activity deposited in a marine environment and of composition that alternates between mafic (rich in iron, magnesium and calcium) and sialic (rich in silica and alkali). This association of amphibolites and leptinites constitutes a large part of the Strona-Ceneri Marginal Zone and is known as the LAG (Leptinite-Amphibolite Group) throughout the European Hercynian orogenic belt.

Near the commemorative plaque, it can be seen that many bands exhibit a variation in grain size: the grain size decreases towards the part of each layer that was originally the highest. Radiometric dating indicates a formation age of approximately 550 Ma. Subsequently, these volcanic rocks also underwent Hercynian metamorphism (340 Ma). The main layers retain a decrease in grain size from the bottom to the top, known as ‘gradation’, which indicates the original stratification of the sediments.

From a morphological point of view, it is clear that the watershed crossed by the road is linked to the presence of a zone of fracturing in the bedrock, as can be seen on the banks of the stream.