Cliff material auto appearing on steep slope

Hi. Take a look on this video Creating a quick Unreal Engine 4 desert scene - YouTube

starting with 0:10 you can see, that the edges of the ground ( landscape actor as I assume ? ) automaticaly show a rock material ( cliff material ) on steep area of landscape when this guy starts to work with sculp tool. I have never seen that before - can someone tell me how to achieve such effect ? I will be very grateful.

Thank you :slight_smile: