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
Glacial erratics
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.
- Fair Isle carries red tills that contain millet seed grains typical of Permo-Triassic sandstones but which also occur locally in Devonian rocks (Flinn, 1970). As Permo-Triassic sandstones occur on the sea bed both east and west of Fair Isle, these erratics provide limited evidence for former ice flow. The tills also contain clasts of granite and of rocks of similar lithology to the Spiggie pluton of south Mainland. Gneiss clasts occur which do not match known Shetland rocks and may be derived from Norway (Flinn, 1978).
- South Mainland shows sandstone erratics on the west coast, implying from moving from east to west across the southern spine of Mainland
- Foula has erratics of Spiggie Granite and metamorphic rocks from the Clift Hills
- Granodiorite erratics on Out Skerries were sourced from Dales Voe
- Erratics from the east are found on Esha Ness
- Erratics of gabbro and serpentinite from Unst occur on the northeast coast of Yell
- Erratics of metamorphic rocks from the SE are found on Ronas Hill
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.