Glacially-modified fault-line scarp of the Beorgs of Housetter. Movement along the fault probably ceased millions of years ago but the juxtaposition of resistant granite against sheared schist led to differential weathering and erosion and the formation of a scarp.
What was Shetland like before the Ice Age?
The diversity of the geology of Shetland reflects the bringing together of rocks of quite different age and character by lateral movement along major faults. The gathering of these exotic terranes has left the island group as a series of basement highs with a partial cover of Devonian sedimentary and volcanic rocks. Similar basement blocks offshore are separated by fault-bounded sedimentary basins. This makes for exciting geology and the remarkably varied scenery.
Dislocation of the basement rocks continued after the Caledonian orogeny. There is evidence offshore of fault movement, uplift and sedimentation in the early Triassic, early Jurassic and early Tertiary. Faulting often reactivated pre-existing structures, with the fault marking the eastern edge of the Unst Basin active in Permian and Neogene times. This was not exclusively so because fault-bounded half grabens N of Unst show different trends in the Triassic and Jurassic. During each of these periods and perhaps others, it is likely that there were differential vertical movements between the crustal blocks on what is now Shetland.
The ancient Devonian hillsides can be seen between Lerwick and Sumburgh where rocks which are no more than lithified scree cling to the original hillsides where they accumulated and were then buried. The current level of erosion lies not far below that achieved at the end of the Devonian. Some features are exhumed, such as the Spiggie basin, overlain to the east by Old Red sandstones.
The Spiggie depression - an ancient topographic basin developed on granodiorite
The re-exposure of the basement rocks from beneath a cover of Devonian and younger sediments appears to be a relatively recent event in geological terms. The sliver of Lower Cretaceous sediments on the low relief East Shetland Platform indicates planation of the basement terrain at that time (Johnson et al, 1993). Planation may have extended across Shetland also. Younger Cretaceous sediment, including the Chalk, probably once covered Shetland but these rocks were stripped in response to uplift in the Palaeocene. The volume of sands in the Palaeocene-Eocene fans on the western edge of the Viking Graben require, however, much deeper erosion and it is likely that Devonian-Triassic cover, plus locally significant depths of basement, was lost to erosion at that time (Morton, et al., 2004).
Looking north from Ronas Hill
The reconstruction of the preglacial relief of Shetland is made difficult by the ubiquity of glacial erosion and by the limited extent of the land area above sea level. Remnants of little-modified preglacial relief can be identified widely where they have been incised by, and so predate, glacial valleys. More generally, there is a strong association between lithology and structure and relief position on Shetland which can only be understood in relation to long-term differential weathering and erosion. On North Roe, for example, the contrast between the low domes of Ronas Hill and the ice-moulded granite plateau to the north is largely a reflection of the more intense fissuration of the northern part of the granite. Similarly, the fault line scarp of the Beorgs of Housetter is a product of differential weathering and erosion between the granite and the pelites to the east (Coque-Delhuille and Veyret, 1988). The survival of fragments of older relief between glacial valleys and fjords allows estimation of the depth of glacial erosion in the valleys.