The main character is a cross country ski hobbiest. He is an experianced rock climber and has climbed ice walls.
He is captured in winter country and there has been a snow storm or two during his capture.
He locates his equipment and escapes into the snow, heading into the mountains. If he gets over the ridges, he can reach safety.
A good time after he escapes, persuit starts.
The scene in question comes to his final escape. I have him climbing up a ridge, digging his way through the base of a cornice (the snow overhang that builds up on the leward side of the ridge) and at the top, breaks it free to fall on his persuers, giving him a safe and easy escape. The thing is, is this posssible? Can he dig through a cornice without causing it to collapse, and then dislodge it from the top without being dragged down with it?
I figure he would use the back edge of his ski to cut the snow to the rock.
Thanks ahead of time.
However, I'm not sure how well snow would hold up under his weight, if he is indeed trying to destabilize it and burrow through it at the same time. I'm having a tough time visualizing this, but I'd imagine even stratified snow (snow with many layers from multiple precipitation events, and likely ice layers as well) would collapse under his own weight long before he'd have the chance to make it to the other side. That's assuming, of course, that he's deliberately setting up the bad guys.
If I were you, I would concoct some kind of ingenous way of getting up the cornice without destabilizing it (critically, at least), yet have your MC rig up a catalyst for the avalanche once he reaches the top/other side. Say, a crude explosive cobbled together from the gunpowder charges of several flaregun cartridges. You'd probably have to set off the charge inside the cornice to knock it off the ridge, but it sounded like he'd be tunneling through it in some fashion, anyway. And as your MC would surely know...snow isn't all that hard to get moving. And once it is, it's like a freight train rumbling down on top of you, only twice as fast and half as loud.
If you make your MC confident, your facts believable (in other words, based on some precedent), and your exposition intense, the reader will go along for the ride.
Inkwell
-----------------
"The difference between a writer and someone who says they want to write is merely the width of a postage stamp."
-Anonymous
Now, if you say that he's desperately trying to get past an unstable cornice by going technical on the underlying face, and then some yahoos come up and start shooting at him, causing the cornice to break away (which he only narrowly survives due to being bolted to the rock), then it doesn't require him to have supernatural control over unpredictable events. It's an accident. He got lucky.
The other idea is he keeps going, escaping, and as the other group tries to get through the cornice, it gives way, taking them with it.