I used this technique to make a river bank but it could be used anywhere you want to show rock stratta. the best part is that it is so EASY!! Materials needed: blue or pink foam, utility knife, paint. That's it.
First, carve the foam to the general shape you want. Pic #1
next use your knife to make horizontal slices in the foam. the more the merrier. After a while it will start to pull little pieces out (which is what you want) If the knife starts to "rip" the foam, change blades. It should look something like Pic #2
I used a dark brown paint first on the recessed areas and then a lighter tan color to highlight the surface areas. Pic #3
Other colors could be used like shades of gray
