The Outer Hebrides have a long and varied geological history with a considerable variety of rock type and structure.
The predominant rock type is a metamorphic rock known as Lewisian gneiss, which takes its name from the largest island area in the archipelago. Geologists have dated this rock at nearly 3000 million years, making it the oldest rock formation in Britain. The gneiss, which is largely composed of a coarsely crystalline mixture of quartz, feldspar, mica and hornblende, believed to be mainly derived from igneous rocks that have been repeatedly subjected to intense heat and pressure.
Unaltered sedimentary rocks are limited to an area north and east of Stornoway. These are mainly sandstones and conglomerates and are believed to date from Triassic times (about 200 million years). No fossil evidence has been found.
Igneous rocks are mainly found as intrusive dyke swarms, many of which form an extension of the Tertiary igneous complexes of Skye and Mull. Most of the dykes are composed of basalt and are aligned in a NW-SE direction. Their relative softness compared with the country rock has resulted in the erosion of a large number of precipitous coastal inlets or geos. The small stacks known as the Maddies that guard the entrance to Lochmaddy harbour are thought to be the remnants of an intrusive dolerite sill.
Evidence of folding and faulting is commonly seen in the gneiss. Ice, running water and the sea have preferentially eroded shatter zones, which have been weakened by dislocation. Marine erosion along a plane of structural weakness has produced the arches and blowhole at Griminish Point in North Uist.
The most important structural feature in the Outer Hebrides is the Outer Hebrides thrust zone. The thrust plane, which marks the boundary between moved and unmoved rock, stretches for almost 200 km from Barra to Northern Lewis. Movement along the thrust resulted in the formation of a wide range of cataclastic rocks. Perhaps the most interesting of these is pseudotachylite, a glassy rock formed by frictional melting, which is well exposed near Greian on the Isle of Barra.
A range of geomorphological features dates from the period known as the Ice Age. The mountain areas of North Harris and South Uist are thought to have had their own small glaciers. Evidence of glacial erosion is well seen in the north-eastern corrie of Beinn Mhor on South Uist and a magnificent example of a truncated spur is found at Sron Ulladale in Harris. Loch Seaforth is one of the finest fjords in North West Scotland.
The Outer Hebrides as a complete structural unit is believed to have been separated from the Mainland in early Pliocene times when, as Western Scotland subsided, the Atlantic waters invaded the area now known as the Minch. However, the division of this unit into the islands of today is thought to have resulted from a rise in sea level at the end of the Ice Age, about 8000 years ago.