Glaciers often created new glens by carving through the pre-glacial watersheds. The Lairig Ghru and Lairig an Laoigh are spectacular glacial breaches through the west-east watershed of the Cairngorm mountains. They were probably initiated by glaciers spilling northwards when their outflow to the south was blocked by ice along the Dee, then later deepened by powerful ice streams within ice sheets moving north and east across the area.
Glacial breaches are formed where repeated periods of erosion have cut through watersheds between valleys. In the Cairngorm mountains, they are shown in a variety of ways:
Examples: Lairig Ghru, Lairig an Laoigh, The Saddle, above Loch Avon.
Glacial breaches are also associated with glacial diversions of drainage. In the Cairngorm mountains, the watershed between the streams flowing to the Spey and those flowing to the Dee has been breached, but the rivers still flow in their original pre-glacial directions; the best examples are the Lairig Ghru and the Lairig an Laoigh.
In some areas, however, the breaching of watersheds has completely altered the directions of river flow. For example, the present headwaters of the River Feshie formerly flowed east to join the Dee, but now turn sharply to the west to join the Spey; the Tarf Water was formerly a tributary of the Dee but has been diverted into the Tilt and now drains towards the Tay; and in the east, the River Avon has been diverted north from the Don to join the Spey.
Examples: River Feshie/Glen Geldie capture, River Avon capture of upper Strath Nethy at Inchrory, and the Water of Caiplich.