[HELP] What exactly needs to change to make terrain impassible?

[shocky27]

Hello,

I have been creating religions, provinces, countries and cultures with no issue. However, when I created terrain that I meant to be impassible, it still shows as a normal city (just with no pops, default plains terrain, no trade good). I can't seem to figure out how to make it impassible, any help is appreciated.

Thanks!