Geology of the Isle of Lewis and The Isle of Harris - Western Isles
Lewisian Gneiss - As well as being spectacularly pretty, these rocks are 3 billion years old, yes they are actually two thirds the age of the Earth (what a thought).
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Isle of Lewis & Isle of Harris - Rocks - Geology of The Western Isles
Lewisian Gneiss was buried a long way beneath the surface for many years For hundreds of millions of years, these rocks were buried beneath the surface which meant that the intense heat and pressures helped to form the growth of the new minerals in them.
It is thought that the original rock was probably sandstones and limestone or igneous rocks like basalt and granite