This is something that comes up a fair bit. I created a quick tutorial on the line art here: How to draw cliffs – 3 different styles | Fantastic Maps

The key to generating depth, is to darken and desaturate (or blue-shift) the lower area, and possibly even slightly blur it to give the impression of great height. Lighten the higher areas and given them the greatest dynamic range (lights to darks). That will bring them into the foreground.