ice flow models

scandinavian glaciation?

 

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

  • a strongly maritime location, providing potential for abundant snowfall and relatively elevated temperatures

  • Shetland is approximately 100 km long and 10-50 km wide, giving a N-S ridge whose maximum height is nowhere above 450 m

  • basement and cover rocks giving a rigid glacier bed are exposed for 40 km west of the axis of the Shetland ridge and 80 km east. Beyond lie thick Quaternary sediments that provided deformable beds for glacier ice to flow across.

  • the continental shelf lies 60-90 km east of Shetland. Deep water would have induced iceberg calving and no ice sheet can have extended beyond this

  • the Scandinavian ice sheet flowed along the Norwegian Trench and extended at times west of this onto the East Shetland Platform

The average glacial conditions are indicated by

  • a general absence of features of mountain or local glaciation

  • orientations of large roches moutonnées that indicate that the ice shed usually lay over the axis of the islands, at least from central Mainland northwards

  • ubiquitous signs of the passage of highly erosive ice, implying glacier ice close to 0ºC at its base

  • 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.