the oldest rocks in great britain are the lewisian gneisses, metamorphic rocks found in the far north west of the island and in the hebrides (with a few small outcrops elsewhere), which date from at least 2,700 ma (ma = million years ago). south of the gneisses are a complex mixture of rocks forming the north west highlands and grampian highlands in scotland. these are essentially the remains of folded sedimentary rocks that were deposited between 1,000 ma and 670 ma over the gneiss on what was then the floor of the iapetus ocean.