Styles of glaciation
The NASA model above is a useful starting point for considering the average glacial conditions on Shetland.
Shetland possesses some singular characteristics which must have acted as strong controls on its glaciation
The average glacial conditions are indicated by
an absence of glacimarine sediments close inshore
moraines systems on the shelf edge and on the fringes of the archipelago
High snowfall on Shetland would allow rapid glacier build up whenever temperatures dipped sufficiently, greatly restricting the time available for local glaciers to shape the landscape and so accounting for the poor development of corries and glaciated drainage basins on Shetland. Initially the corrugated terrain would guide the flow of relatively thin ice. The main discharge outlets for ice would be the pre-existing channels provided by earlier sedimentary basins - the basins of St Magnus Bay, Fetlar-Unst and West Fair Isle - and the soft floors of these straits would allow rapid ice flow. Ross (1996) notes that -100m bathymetric contour is a key boundary around the islands for glacier extent. Global sea level falls to about this level during cold stages and so ice from Shetland can reach this point without calving and losing mass. Using a simple ice profile model for rigid glacier beds, Ross calculates that an ice sheet at this boundary would be 50 km across and just 234 m high. This approximates to the average configuration for ice caps on Shetland.
The moraines on the shelf edge require the development of an ice sheet with a diameter of 150 km. This needs a further drop in sea level, possibly with complex bulging of the crust due to loading by ice sheets in Shetland, Scotland and Scandinavia. This probably requires the onset of full glacial conditions and so relates to glacial maxima. With these limits, an ice sheet must cover all of Shetland, including the summits of Foula and Ronas Hill. Ross (1996) estimates 1200 m of ice cover but this is probably too high as the soft-floored basins would act to draw down ice towards ice margins in the north and west.