hercufles: I hope this helps
That is definitely a bug. I can't find anything about this on the internet, so I can only think of two things to try:
1) Reload from a save file from before you answered the riddle, if you have one. I seem to remember encountering this bug once before, and that this fixed it for me.
2) Try to jump towards the door and click it to zone into the troll caves, then exit back to the gorge. Depending on how the game saves its state, the stairs might be placed correctly when you leave. If not, though, and you still fall in the hole, don't proceed this way. You do need to return to the gorge later.
If neither of these work, you may just have to start over. :/