Shetland from space. Image courtesy of NASA Earth Observatory
Shetland geology map
Scalloway. The firth has been excavated along the line of the Nesting Fault and the Laxfirth Limestone between ridges developed on phyllite and intrusive igneous rocks
Spiggie lowland, a result of preglacial weathering and glacial erosion
Highly fragmented rocks adjacent to the Melby Fault in Braewick Bay
Rocks and relief
Appalachian topography: a landscape of ridges and valleys developed in response to differential weathering and erosion of alternating bands of weak and strong rock
The terrain of Shetland often shows a pronounced NNE-SSW grain which derives from major structural trends. The major faults - the Melby, Walls Boundary and Nesting Faults - provide lines of breakage and weakness that have often been exploited by erosion to create major valleys and the arms of firths and voes. A series of important thrusts also occur, which have brought together rocks of markedly different resistance. Finally, the rocks themselves have been folded and tilted to juxtapose narrow bands of very different lithology, often with near vertical foliation.
Near vertical phyllites of the Clift Hills
These differences have been exploited by weathering and erosion over the last few million years. The susceptibility to chemical weathering has left biotite-bearing rocks, such as the Spiggie Granite, as topographic lows.
Other rocks have been intensely sheared, especially next to faults (Flinn, 1977), greatly reducing their resistance to both chemical and mechanical attack. These lines of weakness have been exploited by preglacial rivers and the glaciers of the Ice Age.
The resultant Appalachian relief of east Shetland is one of the best examples of its kind in Scotland, comparable to the terrain flanking the Ladder Hills in the northeast Grampians. The ground west of the Walls Fault has fewer linear features and the grain of the landscape is more varied, a reflection of the numbers of granite and diorite intrusions and the tight folds of the Devonian sandstones and volcanic rocks. The rounded hills of eastern Unst and Fetlar are developed on large masses of serpentinite and gabbro, separated by phyllites and schists.