The plain of Caithness has been significantly modified by the passage of ice sheets to give a strongly lineated terrain. The main ridges run SE to NW, parallel to ice flow, and are separated by shallow valleys and depressions excavated in zones of rock weakness. Locally, as around Ulbster where the ice had to rise ~230 m to leave the floor of the Moray Firth, the ridges are ice-roughened, with numerous crags, bedrock surfaces and elongate lochans. More generally, the gentle dip and closely spaced bedding of the flagstones give a subdued form to the ridges. The depressions are rock gouges now smothered by till and include valley-fills 20 m deep at Wick. The streamlining of the terrain is picked out by the orientation of lakes filling rock basins on the plain of Caithness. It is noteworthy that this alignment continues into Sutherland, requiring that the inland and shelly till ice flowed side by side to cross the north coast.

Satellite image of the plain of Caithness south of Thurso. Oriented lakes infill glacially-excavated rock basins. North lies at the top left corner of the image
Large glacial erratic near Reay

It is only on the basement rocks and conglomerates that classic ice-roughened terrain is developed. The widely spaced vertical joints in these hard rocks allow plucking to remove blocks on the lee sides of rock bumps, allowing the formation of roche moutonnées. Good examples occur at Cnoc Spardàin in the lower Langwell Water valley and south of Reay. An outstanding example of ice-roughened scenery is found just over the county border at the mouth of Strath Halladale, where the granite hills show a succession of cliffs facing north. On the Devonian sedimentary rocks, the resistant bands tend to form rock ribs, as at Ulbster.

Cnoc Spàrdain

© Copyright Richard Webb / Geograph

Ice-roughening near Ulbster, with resistant conglomerate bands picked out as rock ribs on the skyline

Erosion has not been effective everywhere. Southwest Caithness shows few well-developed landforms of glacial erosion. Instead, exposures of deeply weathered rock occur commonly beneath till and the preservation of tors and tor roots on hills indicates negligible glacial erosion. Here, the ice was frozen to its bed and non-erosive.

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