Western Isles Geology
Western Isles Geology
The Outer Hebrides comprise a 210km archipelago off the north west coast of Scotland. It consists of hundreds of islands, the major ones being Barra in the south, South Uist, Benbecula, North Uist, and the largest of the group, Lewis and Harris, in the north.
The islands comprise an incredibly variable scenery, from the mountainous areas in North Harris and the eastern seaboard of Uists and Benbecula, to the flat mid-island regions that are covered with numerous locks and peat bogs. The coast is typically rocky in the east, but with long white sandy beaches in the west, that blend inland into a gentle, flat strip of fertile duneland or machair The recent glaciations, tree removal by our ancestors, and the strong Atlantic winds have ensured a paradise for geologists, with no lack of inland and coastal exposures to study.
Except for a small area around Stornoway the main town on Lewis, the islands are composed of an assemblage of rocks collectively referred to as Lewisian. They contain some of the most ancient rocks in Europe, that had once formed the basement of a large North Atlantic continent, now split up, but also exposed today in Greenland and eastern Canada. The oldest ones date from a period 2,800 million years ago, referred to as Archaen, which is itself a section of time in the earth's early history called Precambrian.
The characteristic rock is referred to as gneiss; a very hard crystalline material frequently beautifully banded in various shades of white to black. It originates primarily from volcanic material that had become deeply buried over the aeons, remelted, recrystallised, and thrust back to the surface as a result of a long history of continental movements.
The banded gneiss has been intruded by other rocks during its early history. During a period between 2400 and 2200 million years ago, a basic rock cut through areas of weakness and is now visible as very dark brown or black bands, often many metres in thickness, cutting through the gneiss. They are geologically referred to as Scourie dykes, named after the preceding time zone in the Archaen period. These should not be confused with a more common, similar colour and rock formation, originating from the much more recent volcanics of the Inner Hebridean Islands, only a mere 60 million years ago.
Particularly interesting to geologists is a suite igneous rocks which 2,200 million years ago intruded an area, thought to be something similar to a buried volcanic, island arc. The resulting assemblage was altered by heat and pressure, a process called metamorphism, and is now exposed on the southern part of South Harris. One of these rock types, a light coloured crystalline rock called anorthosite, is the major constituent of the hill called Roineabhal in South Harris, and the site of a controversial, planned super-quarry in the region.
A pink granite and a similar more crystalline version, called pegmatite, can be seen in various parts of the islands, but forms the basis of the mountainous Uig Hills in the south west corner of Lewis. The spectacular rock formation occurred at the end of another geological period, called Laxfordian, about 1700 million years ago.
At the end of this granite intrusion period there took place a phase of thrusting which was to shape the region until the present day It created the Outer Hebridean thrust zone, which follows closely the eastern seaboard of the islands. It is also thought to be the source of the fault which boundaries the western side of the geology defining the Minch, the area of sea separating the islands from mainland Scotland. Rocks demonstrating the different levels of shearing, including Mylonite and Pseudotachylite, can be found in the Island's thrust zone.
The oldest rocks in Great Britain, whose age defies imagination promises to fascinate all levels of amateur and professional. Their variety easy access and sheer beauty will entertain all visitors.
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