The northern border of Trollheimen towards Surnadalen has a complex geology with Cambro-Silurian, Eo-Cambrian (?) and Precambrian rocks folded three times during the Caledonian orogeny. The first recognizable folds, F1, are isoclinal slip folds associated with a pervasive axial plane schistosity and lineation. The Surnadal syncline and other major recumbent folds in the northern part of the area belong to the first phase of folding. The second folds, F2, are rather tight flexural-slip folds, while the third folds, F3, are open flexural-slip structures. A regional dome structure with Precambrian granitic gneisses in the core occurs in the south-western part of the area.
Three fold phases in the northern part of Trollheimen in the Norwegian Caledonides