Defining area's on a model/terrain as impassable

On a terrain model or heightmap how do u define areas that a model should not be able to simply climb e.g. a steep cliff face.

what about making an invisable mesh above this area which gets the physical behaviour of a wall?

I should have added "or not easily passable" to the title

yeah I can do that especially if u don't want the character to go there, but it can get tedious on complex terrain plus on short ledges I would want to detect them and do a climb/jump animation. I'm also don't want  "hard limits" as in "with a little effort area can be doable"

i still dont really get your idea and i m not really in stuff like collision yet but i do my try anyway ^^

in some games a charactar slides down if a hill is too steep. is this what you mean?

afaik itis possible to get the polys an object collids with so you could analyse the normals of these polys and lets say that if the angle between this normal and your groundnormal is bigger than 40

yeah that's what I'm after not quite sure how to do it, jme has a rotateToNormal method I can gage ideas from that for the normal direction but the gravity/sliding stuff :?

You could also use the X and Y coordinates to form an area where players should not go.

The scene is an Island, due partly because I didn't really plan on having "hard" limits per say as much as physical limits that feel like it.

this can be done in several ways.

easiest would be adding an invisible object around the area.

or u can design ur terrain good enough that it is surrounded by mountains or deep valleys so the collision detection for passable will return false since its too steep.

thanks for the advice I'll put some thing together tonight heading of to a wedding in a few does any have a good fake physics gravity/sliding example they're will to share or ideas, the math stuff not really my thing