Best basing material for terrain boards - Personal favs?

Been experimenting for a while now with different materials and just wondered what everyone's favourite material is for basing terrain boards?

More specifically for things like earth/sand/wasteland type terrain rather than flock and static grass.

To help with context, I've been using chinchilla dust for finest>beach sand>kitty litter>bits of cork (in that size order) which works reasonably well but i find there isn't much variation between the chinchilla dust and the sand. I feel there must be better 'first layer' materials out there.

Be interested to find out what everyone uses.