The Isorno Zone (Western Lepontic) corresponds to the Bosco and Bombogno zones in the southern part of the eastern (upper) limb of the large polyphase Alpe Bosa-Wandfluhhorn Fold (located between the Maggia and Antigorio valleys), where these two zones cannot be distinguished. It consists of a very heterogeneous series of para- and orthogneisses, amphibolites, meta-gabbros, ultramafites, calc-schists and marbles that underwent an Oligocene amphibolite facies metamorphism.

Corps ultramafique de la Zone de l'Isorno, resp. de la Zone du Bombogno.