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The Highlands lie to the north and west of the Highland Boundary Fault, which runs from Arran to Stonehaven. This part of Scotland is largely composed of ancient rocks from the Cambrian and Precambrian periods which were uplifted during the later Caledonian Orogeny. Smaller formations of Lewisian gneiss in the north west are up to 3,000 million years old and amongst the oldest found anywhere on Earth. These foundations are interspersed with many igneous intrusions of a more recent age, the remnants of which have formed mountain massifs such as the Cairngorms and Skye Cuillins. A significant exception to the above are the fossil-bearing beds of Old Red Sandstones found principally along the Moray Firth coast. The Great Glen is a rift valley which divides the Grampian Mountains to the southeast from the Northwest Highlands.
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