The Mont Mary Nappe is a Salassic tectonic unit constituted by two crystalline basement units: a Lower Unit mainly comprising pre-Alpine amphibolite-facies paragneisses with minor amphibolites and orthogneisses and an Upper Unit comprising pre-Alpine amphibolite- to granulite-facies paragneisses with mafic and carbonate rocks.

The Upper Unit of the Mont Mary Nappe consists of high-grade pre-Alpine paragneisses (kinzigites auct.), amphibolites, marbles and minor spinel-amphibole harzburgites. It is similar to the Valpelline Unit of the Dent Blanche Nappe.

The Lower Unit of the Mont Mary Nappe mainly comprises pre-Alpine amphibolite-facies paragneisses and minor amphibolites and orthogneisses.