Folding of rocks has two main effects when denudation shapes the rocks into landforms. Firstly, the rocks are tilted and warped. Beds of resistant rock give persistent structurally-controlled slopes in the landscape. Cuestas may develop where rock resistance is unequal, with the resistant bed forming an inclined slope parallel to the dip of the rock (dip slope) and a strike-aligned scarp. Secondly, the flexing brings the bed of rock into tension or compression. Up-folds or anticlines cause stretching and fracturing of rocks at their core, allowing more rapid erosion and breaching of the anticline. Down-folds or synclines have closed fractures and tend to resistant to erosion, leading eventually to the base of the fold forming a hill, a perched syncline.

Anticline

Syncline
Monocline
Cuestas developed on tilted beds of rock of unequal resistance. Differential weathering and erosion leads to the development of scarp and dip slopes and strike-aligned trunk streams
Birsay. View E across the breached syncline that forms the central depression of West Mainland

The Devonian sandstones of 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. Synclinal ridges include the Tooin ridge of West Mainland. Perched synclines are well displayed on the elongate hills of Eday, with the central trough of the Sound of Fara following the core of the anticline.

Main fold structures on Orkney (Godard, 1956)

Preglacial Landscapes

  • Faulting and landforms

    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. ...

  • Preglacial drainage

    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.

  • Erosion surfaces

    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.

  • Sub-Devonian landsurface

    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. ...

  • Tertiary of the northern North Sea

    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. ...