The wide corridor that descends (diagonally!) to Loch Avon from the top of Stag rocks, and is almost a mirror image of it's neighbour Y Gully. It is steepest at the top, but entry to the gully is often possible on skiers right. Once out of the gully, the snow filled debris runnel often offers a run almost down to the loch itself.
It is very reliable, and popular as a result. Often confused for the nearby Y Gully, and many have skied it thinking they are doing Diagonal Gully. The easiest way to tell is if you can see the head of the loch looking down the gully, you are standing at the top of Diagonal Gully!
Approximate Start Height: 1040m
Approximate Descent: 290m
General Aspect: South
Climbing Grade: I
Notes: Fairly easy for Grade I
Excellent film of descent of Diagonal, Castlegates and 0.5, Feb 2013