The majority of Orkney's Sedimentary Rocks are Middle Devonian in age - Eifelian and Givetian. Upper part of Hoy and Upper Eday Sands edge into the Upper Devonian Frasnian.
Rifting in the Northern North Sea commenced during the early Triassic, peaked during the late Jurassic, and terminated by the late Cretaceous. The Viking Graben can be seen as a failed arm of proto-Atlantic rift. Almost throughout this period, the Shetland area formed an area of positive relief.
The Eday Group comes from two major river systems flowing into the lake basin, one flowing southeast over Eday and a second much larger one flowing northwest over Dunnet Head (Caithness), Hoy and the South Isles. The alluvial plains from these river systems overlapped and merged in the East Mainland and South Isles.
The Orcadian Basin formed in response to extensional tectonic forces and consequent major growth faults with the sediments deposited in a series of tilted "half grabens". The Orkney Flagstone group is 752m thick, and consists of grey and black thinly bedded flagstones clearly laid down in an extensive freshwater lake.
The oldest sedimentary rocks in Orkney are probably of Lower Devonian age. They outcrop to the north and east of the Yesnaby exposure of basement rock.
The oldest rocks in Orkney are the granite gneisses and schists exposed in West Mainland at Yesnaby, Stromness and on Graemsay (Strachan, 2003). The absolute age of the Orkney basement rocks is unknown. The rocks resemble migmatised Moine rocks found within the Kirtomy Nappe above the Swordly Thrust in eastern Sutherland.