The last ice sheet over Caithness

In Caithness, there was much debate in the 1990s as to whether the last ice sheet covered the plain. A basic difficulty remains the lack of finds of organic material, deposited in ice-free intervals, from below the shelly and inland till sheets. Such finds are common in Buchan and have allowed different phases of glaciation to be dated. Instead, early results In Caithness from the analysis of the decay of amino acids in shells sampled from the shelly till indicated that most of the shells being older than 30-50 ka – dating from before the last glaciation. This encouraged the view that Caithness lay beyond the maximum limit of the last ice sheet, despite the lack of evidence for the supposed ice margin on the ground in the form of large moraine systems.

Recent advances in cosmogenic dating have finally allowed this question to be answered. The last ice sheet in Scotland built up after 32 ka and reached its maximum extent before 22 ka. The ice margin then retreated, with the coast of Buchan becoming ice-free at 18 ka before a final major advance was followed by disintegration and melting at 15 ka. The last glaciation of Caithness can be viewed against this background as a sequence of events. There can now be little doubt that the whole of Caithness and Orkney were covered by the last ice sheet.

The different beds of boulder clay and their interbedded helps to give is a picture of the sequence of glaciation.  In south-east Caithness, there is clear evidence of an initial expansion of inland ice. Tills rich in erratics of igneous rocks occur at protected sites on valley floors at least as far east as Latheronwheel and Leavad (L) and perhaps extend to Watten (W). Deposits of till near Berriedale indicate that the ice surface lay above 250 m. This ice movement may have reached Drumhollistan, near Reay, indicating extensive ice cover in northern Caithness. This ice flow path predates the Last Glacial Maximum.

At a later stage ice from the Moray Firth forced its way on to the plain of Caithness, shouldering aside the inland ice which was diverted to flow northwards. The distinctive shelly till has a clear western boundary (green dashed line). The pattern of striations mapped by Peach and Horne indicate that the ice covered the entire plain, including hills almost 300 m high and then crossed Orkney. The high-level erratics on Scaraben and the glacial modification of tors on Morven and its satellites indicates an ice surface above 600 m. Omand (1973) points out that the last striae on the east coast run northwards and may link to a late movement of ice along the Moray Firth margin which deposited granite-rich till near Wick.

The final events during ice retreat relate to readvance of inland ice on to at least the edge of the plain of Caithness at a time when the Moray Firth ice stream was already partly confined within the Firth. Well-developed moraine systems in Strath Halladale and Strathmore record a final push of ice from inland but it is not clear if these ice movements relate in any way to those which formed other moraines near Wick and Halkirk.

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