They range between granite gneiss to mylonite. In the area between the two complexes, there is some amount of gabbro. At the lowest point, the tunnel runs through a section of phyllite, which continues to be the dominant species until the tunnel reaches Otterøya.
They vary in composition from granite gneiss to mylonite. In the region between the two complexes, there is some presence of gabbro. At the lowest point, the tunnel passes through a section of phyllite, which remains the dominant species until the tunnel reaches Otterøya.