The Daal, Foula, a glacial breach on Foula. Image courtesy of Andy Gear.
A glacial valley tends to have a parabolic shape in cross section that is efficient for the evacuation of varying volumes of ice (Sugden and John, 1976). At the close of phases of glaciation, the basic shape is modified by glacial and glacifluvial deposition to give a flat or terraced floor. Glacial valleys may meander gently but they have been straightened, widened and deepened from their preglacial fluvial precursors.
Looking towards Colla Firth. A glacial valley with a broad floor and relatively gentle sides that reflects the low rock mass strength of the flaggy psammites of the Whiteness Division
On Shetland, the valleys are relatively shallow, reflecting the limited relief of the landmass. The highest cliffs on valleys sides occur where the valleys has been cut through a ridge of high ground, as on Ronas Voe beneath the cliffs of The Brough, or where the ice has flowed parallel to the structural grain, as on the flanks of Weisdale Hill.
The degree of valley connectivity is high, with glacial valleys separating hill masses, as on Whiteness. This compartmented relief is perhaps best expressed offshore, with the isolation of islands, such as Whalsay and Vementry from the neighbouring landmass. A number of important through valleys occur, notably The Daal on Foula. Breaching of watersheds and low ridges has been almost achieved elsewhere, with, for example, Quey Firth almost connecting the east coast and west coast via Ronas Voe. The most famous example is the isthmus of Mavis Grind.
Valley orientation is closely tied to the presence of structural lineaments. The valley that connects Burra Firth, Loch of Cliff and Loch of Whatlee on Unst is fault-guided and shows 40-50m of glacial incision along its length. The Walls Boundary Fault and the Nesting Fault also both exert a strong influence on valley alignment. In zones of knock and lochan terrain, the deranged drainage system reflects the significance of ice flow in exploiting joints and other minor lines of weakness.