A crag (sometimes spelled cragg, or in Scotland craig) is a rocky hill or mountain, generally isolated from other high ground. Crags are formed when a glacier or ice-sheet passes over an area that contains a particularly resilient chunk of rock (often a granite plug or some volcanic structure). The force of the glacier erodes the surrounding softer material, leaving the rocky block protruding from the surrounding terrain. Frequently the crag serves as a partial shelter to softer material in the wake of the glacier, which remains as a gradual fan or ridge forming a tapered ramp (called the tail) up the leeward side of the crag.

In older examples, or those which are latterly surrounded by the sea, the tail is often missing, having been removed by post-glacial erosion.

Examples of such crag and tail formations include:

• The Castle Rock in Edinburgh, Scotland, the rock on which Edinburgh Castle stands.
• Salisbury Crags and Arthur's Seat, Edinburgh
• Three in or near Stirling, Scotland, including the rock on which Stirling Castle stands.

Similar formations to the Crag and Tail are:

• Drumlin
• Roche moutonée