The Mergoscia Zone is a heterogeneous tectonic unit of the Western Lepontic (Toce Culimnation). It is composed of ortho- and paragneisses, metapelites, calcschists, metabasites and meta-ultrabasites. Both metabasites and meta-ultrabasites contain rare relics of high-pressure metamorphism (retrogressed eclogites and garnet-metaperidotites). This zone, displaying a high degree of deformation, is also interpreted as a mélange rather than as an originally stratified framework.

Micascisti eterogenei, a due miche con quarzo e plagioclasio, spesso a patina bruno-rossiccia.

Gneiss a biotite, feldspato alcalino ed oligoclasio, eterogeno a tessitura nodosa-venosa, a carattere migmatitico.