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Geology

The main islands of the Outer Hebrides and the north-western part of the Scottish Highlands are made up of some of the oldest rocks in Europe, known since the late 19th century as Lewisian gneisses. The name describes a series of metamorphic rocks formed by intense pressure and temperature over a period of 1500 million years. Most of these gneisses started off as igneous rocks, such as granites and gabbros, formed by the cooling and crystallisation of magma nearly 3000 million years ago. These original rocks were then destroyed when they were buried, reheated and subjected to great pressures in the earth's crust, eventually forming the metamorphic gneiss complexes we see today.

Lewisian gneiss is characterised by narrow, alternating bands of contrasting colours. The paler bands, which are typically pale grey or pink, are made up of crystals of quartz and feldspar, whereas the darker green and black bands are largely made up of minerals called amphiboles. Examples of this striation can be seen in the exposed boulders on the western beaches of both Harris and Lewis as well as in many of the stones at Calanais.