The Highland Boundary Fault is a major fault zone that traverses Scotland from Arran and Helensburgh on the west coast to Stonehaven in the east. It separates two distinctly different physiographic and geological terrains: the Highlands from the Lowlands, and in most places it is recognisable as a change in topography. Where rivers cross the fault, they often pass through gorges, and the associated waterfalls can be a barrier to salmon migration.