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.
The rock types in this area range from granite gneiss to mylonite. In between the two complexes, there is some amount of gabbro present. At the lowest point, the tunnel passes through a section of phyllite, which remains dominant until it reaches Otterøya.