A glacially eroded trough that cuts through a ridge and so breaches a former watershed. It is formed when the outflow of a glacier (or ice sheet) is impeded, its thickness consequently increases, and ultimately a new escape route (the breach) is exploited. This process is called ‘glacial diffluence’ when a single glacier spills out of its valley, and ‘glacial transfluence’ when several breaches are formed due to the accumulation of a large ice sheet. The many breaches through the western Highlands of Scotland are due to the accumulation of transfluent ice east of the main watershed.