In Burtness Comb, a prominent bank of debris runs along the outer flank of the upper cirque, then turns down into the lower cirque, ending in a block strew at the intake wall. It marks the course of a rock avalanche which fell from Grey Crag on the summit rim of High Stile, and was channeled along the foot of the glacier some 11,000 years ago. It is unique in the Lake District.