Our understanding is poor of the timing of the major events that have shaped the terrain of Orkney. This uncertainty reflects the fact that Orkney has been an area of net erosion for much of the last 60 million years. The general sequence of events can be pieced together using evidence from sediments in the North Sea (Johnson et al, 1993) and the continental shelf west of Orkney (Stoker et al, 1993).

We can view the landscape history of Orkney in terms of three main periods, each much shorter than its precursor:

• the Tertiary (65-2.5 Myr) saw the shaping of the main preglacial features of the terrain, the major hills and valleys
• the Pleistocene (2.5 Myr-10 kyr) brought dramatic shifts in climate, with periodic glaciation. Glacial erosion excavated the deep firths of the archipelago
• the Holocene (the last 11.5 thousand years) when climate warmed and sea level rose to its present level

The geological history is much older. 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.