Geology
Most of the islands have a bedrock formed from Lewisian gneiss. These are amongst the oldest rocks in Europe, having been formed in the Precambrian period up to three billion years ago. 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 and intruded by later basaltic dykes and granite magma. The gneiss's delicate pink colours are exposed throughout the islands and it is sometimes referred to by geologists as "The Old Boy".
Granite intrusions are found in the parish of Barvas in west Lewis, and another forms the summit plateau of the mountain Roineabhal in Harris. The granite here is anorthosite, and is similar in composition to rocks found in the mountains of the Moon. There are relatively small outcrops of Triassic sandstone at Broad Bay near Stornoway. The Shiant Isles and St Kilda are formed from much later tertiary basalt and basalt and gabbros respectively.
Read more about this topic: Outer Hebrides