Dark erratics on the red granite of Ronas Hill. Locally derived?
Erratic on Esha Ness identified by Allen Fraser as originating from Bight of Stavaness, North Nesting
Boulders from the Bight of Stavaness - is it the same rock?
Glacial erratics in
Definition: An erratic is a boulder transported and deposited by a glacier having a lithology different than the bedrock on which it rests. Erratics are useful indicators of patterns of former ice flow.
Steens o Stofast, Lunnasting
The diverse geology of Shetland presents many possibilities for the study of directions of erratic carry by glacier ice. Erratic clasts ranging from pebble to boulder size occur at the surface, beneath peat and in glacial tills. On lower ground large blocks have been removed from fields to improve land for agriculture and the walls of the field boundaries give a good indication of the large erratic content of local tills. Perhaps the largest erratic on Shetland is the Steens o Stofast on Lunna Ness - but Allen Fraser notes that the Steens are composed of Valayre Gneiss and lie on bedrock of the same composition. The Valayre Gneiss outcrops across no more than about 300m on this part of Lunna Ness, thus if the Steens cannot have been moved further that a few 10s of meters.
In contrast to Orkney, there are no rafts of glacio-marine or marine mud known on Shetland. This implies no significant movement of ice across the bed of the northern North Sea towards Shetland and represents powerful evidence against the direct flow of Scandinavian ice to Shetland.
Ross (1996) reviews existing evidence of directions of erratic carry on Shetland.
Although directions of erratic carry often conform to flow patterns within an ice cap with an ice shed running roughly N-S, other erratic trains imply topographically constrained flow beneath thin ice masses during deglaciation. Ross finds no convincing evidence from patterns of erratic distribution for the former presence of Scandinavian ice on Shetland.