Hadn't seen any signs of instability and the new snow seemed decently bonded on the skin up, so decided to ski something steep-ish. Made two ski cuts at the top and nothing moved. Half way down the run the snow had other ideas and a point release propagated into a 50' wide slow moving slide that entrained all of the new snow. Tried to ski out of it, but lost both skis. Went for a ride for a couple hundred vert, but was able to stay on top sliding on my butt/back. Was wearing and ready to deploy an airbag, but didn't need to. The slide spread out and came to a gentle stop as the terrain flattened. I came away unharmed. A nearby group of three that witnessed the slide called over to make sure I was ok and no one else was involved, then kindly helped my fetch my skis, which fortunately ran downhill rather than getting buried. I alerted Alta staff and a few skiers starting their tour at the bottom. There was at least one other similar slide nearby by the time I got to the bottom.