The gross form of the Shetlands evolved under non-glacial conditions during the Tertiary. Two elements of the Tertiary environment were important here:
The Tertiary climate was much warmer but less varied than that of the Quaternary. Maximum warmth in Shetland was probably achieved during the Eocene. Vegetation remains and latosols preserved between lava flows in the Tertiary Igneous Province in western Scotland and Faeroe indicate humid tropical conditions. Climates cooled during the Oligocene but remained maritime sub-tropical to warm temperate in character into the middle Miocene. Towards the end of the Miocene, temperatures dropped and temperatures in lowland Scotland remained generally close to today's. North Atlantic cores indicate that relatively short periods of cold affected Scotland during the Pliocene.
The prevalence of warm and humid conditions is significant for the evolution of the relief. Deep chemical weathering is a highly effective process under these circumstances, leading to etching out of differences in bedrock resistance within the landscape. The deep weathering profiles would have been highly kaolinitic, as in tropical climates today and as shown by the composition of contemporaneous sediments in the northern North Sea. These kaolinitic weathering mantles have been stripped away, latterly by ice sheets in the Quaternary.
Shetland occupied a pivotal position in relation to the opening of the north-eastern Atlantic that commenced around 60 million years ago. Prior to this, in the late Cretaceous, the land mass was almost certainly one of low relief, as near horizontal erosion surfaces occur beneath a veneer of Upper Cretaceous rocks on the East Shetland Platform, now submerged. The Shetland landmass was producing little sediment at this time and fine-grained mudstones were deposited in the northern North Sea, probably sourced from nearby Greenland.
Ocean spreading led to regional volcanic activity from western Scotland to Faeroe and Greenland. Shetland was uplifted and probably tilted towards the North Sea. Very large volumes of sand were shed south-eastwards, implying deep erosion of substantial hills or even mountains.
Periods of tectonic calm characterised the middle Eocene and middle to late Miocene. Lesser periods of uplift probably occurred from the Late Oligocene and from the early Pliocene (Stoker et al, 2005) and these phases produced the hilly terrain found on Shetland today.