The geology of Shetland is varied and rather complex, principally as a result of a large number of major north-south running faults, including the northward continuation of the Great Glen Fault, which has brought a large number of different rock types together in a narrow zone.
Shetland displays the geology of northern Scotland in miniature, offering the chance to see ancient Lewisian Gneisses, 1000 million year old rocks of the Moine Supergroup and younger Dalradian rocks (600-700 million year old), in the space of a few kilometres. On Unst and Fetlar there occurs a slice of oceanic crust and upper mantle rocks, forming the ‘Unst Ophiolite’, which adds to Shetland’s extraordinary geological diversity.
In contrast with Shetland, much of the Orkney Islands, are formed of sandstones of Devonian age, covering a period of time between 400 and 360 million years ago. Local variations in the hardness of the sandstones give rise to variations in the local topography, including some striking features, such as the cliffs on the west coast of Hoy.
The Devonian sandstones of Orkney and Shetland were deposited in the Orcadian Basin, a low-lying area into which rivers from the Highlands to the west and south drained. Within the basin there was a large freshwater lake. The lake depth fluctuated and at times of greatest depth, the central portions of the lake became anoxic – deprived of oxygen. It was here that fish carcasses were deposited, to become the beautifully preserved fossil specimens associated with parts of Orkney, Shetland and Caithness.
Both island groups are particularly important for their Quaternary deposits and coastal landforms.
Generally, Shetland shows more evidence of glaciation than Orkney, reflected in a greater occurrence of glaciated valleys and fjords. Much of the inland area of Shetland contains ice-scoured rocky hills and with peat over a generally thin drift cover. On Orkney, glacial erosion has generally been confined to moulding and smoothing the landscape, although north Hoy has prominent glacial valleys and corries.