In Caithness, there has been much debate as to whether or nor the last ice sheet covered the plain. A basic difficulty has been the lack of finds of organic material from below the shelly and inland till sheets that would allow these phases of glaciation to be dated. Instead, first results from the analysis of shells from the shelly till indicated amino acid ratios consistent with most of the shells being older than 30-50 ka. 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. There can now be little doubt that the whole of Caithness and Orkney were covered by the last ice sheet.

The last ice sheet in Scotland built up after 32 ka and probably 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.

In south-east Caithness, there is clear evidence of an initial expansion of inland ice. Tills rich in 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 ice surface lay above 250 m. The extent of this ice movement in northern Caithness is uncertain but it probably reached Drumhollistan, near Reay. Its timing is also unknown but it probably 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.