Main image on top: Dunbeath Water (© Copyright Andrew Tryon /Geograph)

Significance: a record of the sequence of events during the last glaciation

The rivers of south-east Caithness have cut deep, unstable sections in thick boulder clay. The revealed stratigraphy shows us that south-east Caithness straddles the boundary between two till sheets. To the north and east lies the shelly till, deposited by ice moving out of the Moray Firth. To the south and west lies the inland till, derived in part from the crystalline terrain of the Langwell and Berriedale drainage basins. The local stratigraphy records the interactions of the two ice streams that deposited the respective till sheets.

The formal units in the stratigraphy of Caithness were defined by Hall and Whittington (1989). The deposits in the shelly till area were assigned to the Lybster Formation. Only one shelly till unit was recognised, and linked to a type site near Forse where it is 6 m deep. The ridges which divide the Berriedale, Langwell and Dunbeath Waters guided ice flow and so here different till units were named in each valley, all within the Berriedale Formation which equates to the inland till sheet.

Peach and Horne (1881) recognised that ice from inland and Moray Firth competed for control of this ground, with first one ice mass and then the other being dominant. The schematic stratigraphy in the three main valleys supports this classic view. The sequence of events from oldest to youngest appears to have been:

(i) expansion of inland ice to leave brown lodgement tills infilling the floors of the three valleys. This advance reached at least as far NE as Leavad and Latheronwheel. In the valley of the Dunbeath Water, striae, erratics and till fabrics show that ice was moving down the valley.
(ii) encroachment of thick ice from the Moray Firth and diversion of the inland ice. In the Dunbeath Water the inland till was eroded and partly or wholly incorporated into the shelly till. We can see this near Dunbeath Cemetery. The shelly till rises to elevations of over 100 m and the striated surfaces on which it rests show ice moving to the NW. The shelly till deposits stop abruptly but no moraines mark the limit of the drift sheet, as would be expected if this was the edge of the ice sheet. Instead, the inland till appears immediately and its erratic content near An Dùn indicates that the inland ice was now moving from the south or southwest. At this stage two thick glaciers were confluent and the shelly till ice must have been thick enough to divert the inland ice.
(iii) disappearance of the shelly till ice, without significant development of moraines. Local readvance of inland ice at the mouth of Berriedale to overstep the shelly till.
(iv) climate warming, peat development at Wag and Ellanmore, and then renewed cold in the Loch Lomond Stadial (12,900 and 11,700 years ago).

Older deposits may lie hidden along the river valley floors.

Images: Glacial deposits along the Dunbeath Water

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