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.

The colour of the tills on Orkney is largely a reflection of the rocks over which the ice had passed. The red marls and sandstones of the Eday Group give a distinctive red till and erratic clasts of red sandstone are a major component of these tills in the eastern parts of Westray, Rousay and West Mainland. Tills also contain a lesser component of far-travelled material, including exotic stones. On Orkney the most important and abundant erratic material is shelly mud excavated by ice moving across what is now the end of the North Sea. This mixed and ground debris forms the matrix for widespread shelly tills. Many records exist of erratic clasts in tills, with the Memoir mentioning granite, felsite, gneiss, quartzite and schist, all probably derived from the Scottish mainland. Various Mesozoic fossil-bearing boulders and cobbles have been recorded, most notably chalk and chalk flints.

The most famous erratic is the Saville Boulder, first described in 1870, of possible Scandinavian origin. An exciting recent find is a rhomb porphyry from the Oslo region – found by David Leather on Rapness in Westray. It is pictured opposite, together with a rather striking gneiss clast.

An erratic carried by glacier ice from the basement rocks of Graemsay. It now lies amongst locally-derived sandstone and volcanic clasts on the beach at The Flashes, Hoy.

Erratics of rhomb porphyry and gneiss from boulder clay, Stancro

  • During the periods of maximum cold in the Quaternary, major ice sheets covered Scotland. An ice stream hundreds of metres thick curved out from the Moray Firth to cross the plain of Caithness and flow over Orkney towards ice limits close to the edge of continental shelf.

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  • In the gently-dipping sandstone terrain of Orkney, it is often difficult to pick out classic landforms of glacial erosion. Low-lying areas often show a pronounced SW-NE grain to the topography, parallel to the main direction of ice sheet flow. ...

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  • Glacial deposition is largely confined to low-lying areas on Orkney, where thicknesses of till may exceed 10 m. The glacial deposits drape the landscape, smoothing its outlines. Ice-marginal features are largely unrecognised outside Hoy ...

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  • Westray is the furthest northwest of the Orkney islands. The total area is 47 square kilometres, not huge but the irregular shape gives it a long coastline of almost 80 kilometres, a good place to look for glacial striations. The bedrock of the whole of the island is made up of the cyclical Rousay Flagstone Formation. ...

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