Glue several layers of thick insulation foam together with a strong glue like liquid nails. It will help to place a few heavy objects on top of the pile to form a stronger bond.

Use a sharp knife to carve out channels and gouges in the cliff face. Remember to be very careful when using sharp knives. Then use filler to plug the gaps that are visible between the layers.

Once the filler is dry you can texture your cliffs any way you like. Varying sizes of sand and gravel can be glued on with wood glue. For our quick example we just painted the cliff with Codex Grey and drybrushed it with successive layers of Rotting Flesh and Bleached Bone.

Flock that looks like dead or dying grass can be glued on top of the cliff, but this is not necessary. You may want to instead place a few rock outcroppings or twisted trees on top to reinforce the rugged feel of a weathered cliff.

Above are a few more example of different cliff faces. The first one was made using wood chips stacked on top of one another and seated into a foam hill. The cliff on the right was built using the same techniques as the one that was explained in the steps above, but a different cutting method was employed for a strip-mine feel.