The geological history of Shetland spans over half of the Earth's history. Although no rocks remain on land that are younger than the Devonian (~380 Myr), the oldest rocks on Shetland, the Lewisian of North Roe, reach back to almost 3000 Myr.

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

We can view the landscape history of Shetland 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 of Shetland. Glacial erosion excavated the deep firths of the archipelago
• the Holocene (the last 10 thousand years) when climate warmed and sea level rose to its present level