Select areas where value is changing rapidly

[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...)