The high density of slope failures in western Norway is due to the
steep relief and to the concentration of various structures that
followed protracted ductile and brittle tectonics. On the 72 investigated
rock slope instabilities, 13 were developed in soft weathered mafic
and phyllitic allochthons. Only the intrinsic weakness of such rocks
increases the susceptibility to gravitational deformation. In contrast,
the gravitational structures in the hard gneisses reactivate prominent
ductile or/and brittle fabrics. At 30 rockslides along cataclinal
slopes, weak mafic layers of foliation are reactivated as basal planes.
Slope-parallel steep foliation forms back-cracks of unstable columns.
Folds are specifically present in the Storfjord area, together with
a clustering of potential slope failures. Folding increases the probability
of having favourably orientated planes with respect to the gravitational
forces and the slope. High water pressure is believed to seasonally
build up along the shallow-dipping Caledonian detachments and may
contribute to destabilization of the rock slope upwards. Regional
cataclastic faults localized the gravitational structures at 45 sites.
The volume of the slope instabilities tends to increase with the
amount of reactivated prominent structures and the spacing of the
latter controls the size of instabilities.