Select areas where value is changing rapidly

[geobeck]

I'm trying to create a texture mask for a game map, and need to select the cliffs on my terrain. How can I select the areas where the pixel value is substantially different from adjacent pixels by a certain amount?

[Ofnuts]

Without a sample, difficult to give a good idea. Possible solution:

1. Duplicate the layer
   
2. Set the top to "difference" mode
   
3. Start the "Median blur" filter, when its radius becomes close to the "thickness"  of the cliffs, cliffs will appear over a ver dark background. Make a selection from the adequate R/G/B channel (or create  a new layer and massage it first (desaturation, contrast...)