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.

Structural landforms explained
Major faults on Orkney (Godard, 1956)
  • Orkney have been flexed into gentle folds with a N-S trend. Inverse relief is widespread (Godard, 1956), where anticlines and synclines now form low and high points in the landscape. The central depression of West Mainland and other 'brays' follow breached anticlines. ...

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  • Godard has reconstructed the drainage system immediately prior to glaciation by combining to valley systems on land with those submerged by sea level rise on the surrounding sea bed. Glacial erosion has cut through low watersheds to form the archipelago of today.

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  • Definition: a land surface of low relief reflecting long term erosion. The surface may now lie well above sea level as a result of uplift or be tilted or deformed as a result of later tectonics.

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  • Near Yesnaby the ancient surface is more hilly and Devonian conglomerates are seen plastered onto a steep rock wall that now forms the upper part of the sea cliff. ...

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  • At the opening of the Tertiary chalk sedimentation continued on the northern North Sea. Starting around 63 Ma, the East Shetland Platform was uplifted and tilted towards the south-east. ...

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