What do GW use to based scenery?

[Warfare]

As in the title, what do GW use to base their scenery that gives it differing levels (seen in a lot of the lustria terrain and the jungle terrain in the old 40K rulebook? I would normally use hardboard but I have started to work on jungle scenery and a flat base looks pretty unnatural.

[Engelus]

on your hardboard you can get some natural clay (not plastilina/plasticine as that is impregnated with oil and never dries) and smear it on and layer it till you get the desired effect.