The Gruf Complex is a polymetamorphic migmatitic unit of the Eastern Lepontic (Ticino Culmination), interpreted as a lower crust compartment (possibly related to the Adula Nappe) that was dragged up by the rise of the Bregaglia Intrusion. It directly underlies the Tambo Nappe and the ophiolitic Chiavenna Nappe to the north along a steeply N-dipping fault and follows the base of the Bregaglia Intrusion to the south.

Variszische migmatitische und granitische Gneise der Bodengo-Gruf-Einheit östlich der Mera: isoklinal gefaltete quarzo-feldspatische Bändergneise mit mafische Lagen.