Birsay. View E across the breached syncline that forms the central depression of West Mainland
Main fold structures on Orkney (Godard, 1956)
Folding and landforms
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.
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
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.