Large glacial erratic near Reay
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.
Ice-roughening near Ulbster, with resistant conglomerate bands picked out as rock ribs on the skyline
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-NW, parallel to ice flow, and are separated by shallow valleys and depressions excavated in zones of 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 has been exploited to give a subdued outline to the ridges. The depressions are rock gouges now smothered by till and and include valley-fills 20 m deep at Wick. The streamlining of the terrain is picked out by the S-N 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.
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 generate cliffs 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, where there is a marked change in the terrain moving off the Old Red Sandstone. 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.
Erosion has not been effective everywhere. Southwest
Caithness, in particular, 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