The period since the last interglacial has seen many rapid and marked shifts in the climate of Scotland. Although the terrestrial stratigraphic record is still poorly understood, there is growing evidence for the existence of major ice masses during several intervals during the Late Pleistocene. The last ice sheet appears to have covered Orkney to a depth of several hundreds of metres and to have extended north and west to the shelf edge and to have been confluent with Scandinavian ice in the northern North Sea.
The main sub-divisions of the Late Pleistocene are based on the marine oxygen isotope record:
- OI Stage 5e – the last interglacial
- OI Stage 5d – a cold period
- OI Stage 5c – a long temperate period
- OI Stage 5b – a cold period
- OI Stage 5a – a shorter temperate period
- OI Stage 4 – a long cold period when an ice sheet from Scotland appears to have reached the shelf edge and so may have covered Orkney
- OI Stage 3 – a period of cold with brief warm periods, perhaps with smaller glaciers but moraines at the shelf edge have been ascribed to ice sheets during this interval
- OI Stage 2 – a long cold period when the last (Late Devensian) ice sheet covered all of Orkney
- OI Stage 1 – the Holocene, the current interglacial
The last major period of glacial and periglacial activity on Orkney occurred during the Loch Lomond Stadial (11-10 kyr BP).
Orkney is dominated by Devonian sedimentary rocks deposited around 380 million years ago. These rest on basement granites and gneisses around 1000 million years old.