Getting back to this idea:
Originally Posted by Moedred
How about a frozen lake which is revealed to have no water beneath, just a long drop to the rocky ground? I've actually heard of this phenomenon: a reservoir is built and filled with water, trees in the former valley are topped at water level, the surface freezes, and rests on the many treetops when the reservoir is drained. Sequoias would be too far apart, but 100 foot lodgepole pines are plausible. When it starts to collapse and tilt at odd angles, the chase and escape (on aerosani and snowmobile) up the long valley begins.
For visuals, combine Austria's Kolnbrein Dam from Boys form Brazil...
...and Kazakhstan’s Lake Kaindy, formed
naturally from an earthquake in 1911, but trees have been topped at the highest water level just the same. Add some more stumps, a few inches of ice, then take away the water, and have a chase on the surface through the canyon on aerosans.