While moving from the start point on top of a boulder for fallen slab arete to the start point above it for fallen slab while wearing a backpack my foot slipped in the gully to the left hand side resulting in a fall onto my back and a slide I was unable to recover from. This sent me backwards head first off the original boulder. Fortunately I was somehow able to flip in the air and land on my feet leaving me shaken but unharmed bar minor grazes.