5th skier low in the N bowl of Lake Peak. Was ascending on low posts near the end of the uptrack in the bowl. Heard a soft settling sound, a pressure wave ran up the bowl and a 300' W x 500' H area starting slowly moving. It picked up some speed, crossed a lower angle moraine and then picked up speed again on the other side of the moraine. It stopped at the foot of that slope, and I stayed upright-ish and oriented normally the whole ride probably because the lower slope was so moderate . My pole got a bit buried at the end but everything was okay. I went down maybe 450 to 500 vertical feet, and had plenty of time to think how I had badly misjudged the situation. The crown was 300' above the uptrack and varied in height 1 to 4? Feet. Likely a PWL sliding surface. It was a very lucky ending! You can see the slide from U210 below White Pine.