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.

Godard (1956) provides almost the only account of erosion surfaces on Orkney, an extension of his mapping of the preglacial relief of the entire Northern Highlands (Godard, 1965).

A summit surface is recognised on Hoy, including the fragments of high plateau on Ward Hill, Cuilags and Knap of Trowieglen. This surface covers a broad height range from 150-365 m, probably as a result of Tertiary faulting. On North Mainland, flat summits at 170-185 m may represent the last remnants of a lower and later surface, slightly deformed but not significantly faulted. The Lower Surface at 60-100 m is more widespread and also little deformed. It is correlated on the basis of its height and form with the Niveau Pliocene, a surface found around the fringes of the Northern Highlands.

Image courtesy of Alan Moar

  • 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. ...

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  • 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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  • 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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