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.

In 1881, Peach and Horne published the results of the first comprehensive survey of the patterns of ice flow across Caithness. Their accompanying map remains an excellent summary of the general flow of the last ice sheet at its maximum.

Along the western border of Caithness, the distribution of erratics indicated the flow of ice from the west. For example, there is a train of Devonian conglomerate boulders along the lower flanks of Scaraben. There are few, if any records of far-travelled rocks and the debris appears to have been sourced within the metamorphic terrain or from its cover rocks. Glacial erratics have been recorded to a height of 600 m on Scaraben, giving a minimum ice thickness during the last glaciation.

Erratics of granite and Devonian conglomerate

Dry stane dyke near Reay made from field and quarry stones

The Devonian conglomerate at Sarclet, south of Wick. Erratics of this distinctive rock can be traced across the county to Dunnet Head

Possible directions of erratic carry by glaciers for the Pipe Rock of the NW Highlands, the Chalk of the Moray Firth and the granites in a late boulder-rich till near Wick. Superimposed on Peach and Horne’s (1881) map of ice flow patterns.

The Devonian conglomerate at Sarclet

In contrast, the erratic content of the shelly till has long attracted interest for its variety. Aside from the many species of shell themselves, the tills contain an admixture of exotic stones with the dominant Devonian debris. One source of debris has been the floor of the Moray Firth, providing Jurassic and Lower Cretaceous sedimentary rocks. Another source lies eastern Sutherland and Ross-shire and more remote locations in the Northern Highlands. These erratics of quartzite, gneiss (including Inchbae augen gneiss) and granite are typically small and often rounded and may have had a complex history of transport. Some are downright puzzling. The Cambrian Pipe Rock must originally have been sourced west of the current ice-shed. The chalk erratics around Wick require a contrary flow from the southeast or east. The granite debris identified by Jamieson (1866) in an upper till around Wick is more easily explained, as it probably relates to a late movement of ice along the coast at a stage when the ice was of sufficient thickness only to fill the Moray Firth but not rise over the plain of Caithness.

The cliffs just south of Latheronwheel

Devonian conglomerate erratic, from near Berriedale?

These complexities indicate that the  distribution of erratics is a product of many phases of glacial transport. Nonetheless the pattern of carry of local indicator rocks across the plain of Caithness is consistent with the directions of ice flows identified from striae. The distinctive Sarclet Conglomerate provided a  boulder train that can be traced NNW towards Dunnet Bay from its source outcrop on the cliffs on the Moray Firth coast. Equally, Middle Devonian flagstones have been carried onto the Upper Devonian quartz sandstones of Dunnet Head. Peach and Horne’s map remains a good guide to the flow of the last ice sheet.

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