page_title: Gneiss
section_title: Lewisian gneiss
hierarchical_section_title: Gneiss / Types / Lewisian gneiss
caption_attribution_description: English: Road Cutting The rock faces exposed when the new A838 was built have become a mecca for geologists because they show the relationships of the different types of rock in the Lewisian. The oldest rock is the striped grey Badcallian gneiss, which is 2700 to 3000 million years old. Between 2400 and 2200 million years ago these grey rocks were intruded by dykes of black igneous rock, known as Scourie dykes. These rocks were later deformed and again intruded by coarse pink granites and pegmatites, themselves deformed during the Laxfordian event of 1900 to 1700 million years ago.
context_page_description: Gneiss is a common and widely distributed type of metamorphic rock. Gneiss is formed by high temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Orthogneiss is gneiss derived from igneous rock. Paragneiss is gneiss derived from sedimentary rock. Gneiss forms at higher temperatures and pressures than schist. Gneiss nearly always shows a banded texture characterized by alternating darker and lighter colored bands and without a distinct foliation.
context_section_description: Most of the Outer Hebrides of Scotland have a bedrock formed from Lewisian gneiss.  In addition to the Outer Hebrides, they form basement deposits on the Scottish mainland west of the Moine Thrust and on the islands of Coll and Tiree. These rocks are largely igneous in origin, mixed with metamorphosed marble, quartzite and mica schist with later intrusions of basaltic dikes and granite magma.
caption: Dark dikes (now foliated amphibolites) cutting light grey Lewisian gneiss of the Scourie complex, both deformed and cut by later (unfoliated) pink granite dikes
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