One of the major geological features of Scotland, the Great Glen Fault is particularly extensive running SW to NE for a distance of at least 300 miles (480 km). It can be observed from Colonsay in the Inner Hebrides to Shetland in the Northern Isles. The fault splits the Highlands into the Northern Highlands, to the northwest, and the Grampian Highlands in the southeast. The Great Glen Fault is a tear or strike-slip fault with a displacement estimated at 64 miles (104 km). A zone of shattered rock along the length of the fault is prone to erosion and this has given rise to the development of the most extensive valley in Scotland, the Great Glen (Glen Mor).
From Colonsay, the fault clips the southeast edge of Mull before proceeding across the Firth of Lorn, Lismore and Loch Linnhe to Fort William. It continues through Loch Lochy and Loch Ness, past Inverness and then cuts along the northern shore of the Moray Firth, before curving northwards to Shetland, where it appears as the Walls Boundary Fault.
The fault may have developed as long as 1000 million years ago but was certainly active in the Devonian Period (370 million years ago) and reactivated in the Middle Jurassic (170 million years ago). Very limited movement still occurs on the fault, evidenced by occasional small earthquakes.