Fault: a rock fracture across which movement has taken place
Faulting has two main effects on subsequent erosion. Firstly, the line of fracture and movement is a zone of breakage which is often later exploited by erosion to give a valley. Secondly, faulting brings together rocks of unequal resistance and erosion of weaker rocks may leave a scarp along the line of the fault (fault-line scarp).
On Orkney, faults are regarded as ancient and inactive features. A series of N-S normal faults has limited expression in the relief. The displacement of rocks across the faults (throw) is small and so the chances of major differences in lithology across the fault are slight. The NW-SE faults have much clearer topographic expression and include the major fault scarps which face and define the margins of Scapa Flow. The NW-SE orientation of the main firths may also reflect long term erosion of faults by rivers and ice.