key sites

 

 

Dalsetter erratic

Significance: possible evidence of the former presence of Scandinavian ice on Shetland

The Dalsetter erratic is a rounded block of tönsbergite about one metre in diameter. It is part of a field boundary on the east side of the road at HU 402160. In petrology the Dalsetter erratic is similar to the rock of the tönsbergite intrusion near Tönsberg in southern Norway, notably in its texture, colour and its unusual feldspar structure. There seems little doubt that it is derived from its type locality (Le Bas, 1992). As the sole erratic of Scandinavian origin in Shetland, the Dalsetter stone provides critical evidence in support of external ice from Scandinavia crossing the Shetland Islands. Some caution is necessary in interpreting foreign erratics in Shetland because some material may be derived from ship's ballast or from Viking homesteads. There is however circumstantial evidence that the Dalsetter erratic was extracted from till (Findlay, 1926), probably in a roadside pit close to its present location (Flinn, 1992). There is also the possibility of a two-phase transport, with glacial or ice berg transport across the northern North Sea and entrainment by later Shetland ice moving towards the west. A search of the field walls in the surrounding 7 km2 failed to find any comparable stones (Ross, 1996).