When can settlers transform?

[MyOlde]

I've found that settlers can sometimes transform a city's terrain into hills but not at other times. But I can't figure out why this is so or what the condition is that dictates it.

Thoughts?

[Ming]

You can always irigate or mine to change squares that can be changed with these commands. It takes explosives before you can do any of the terreforming commands.

[MyOlde]

Aha! That's great. Thanks, Ming.