About half way up Thunderbolt North Couloir it "Ys" - this is looking up the left fork of the Y to outcropping of rocks. Climb through/around the rocks the best you can before turning slightly right to head to the crest of the couloir. Image taken March 2007 - note that later in the season the left part of the Y is melted out providing class 2-3 scrambling to get over and through the rocks.