Definition of "Flat land"?

[ditchhook]

The game is telling me that Stonehenge must built be adjacent to stone and on flat land. I have one improved (quarry) source of stone within my city boundary, and another (unimproved) just adjacent to my city boundary. Both these sources of stone are adjacent to swamp land inside my city boundary, and the improved (quarried) source of stone is also adjacent to a flatland rice source. I try to chose to build Stonehenge in my city screen, but I am not allowed, because "No suitable location to construct this."

What am I misunderstanding?

 

[Victoria]

The rice is on swamp?
Flatland is grassland, plains and tundra, not sure about ice.
Just drain the swamp.

 

[Pietato]

Just drain the swamp.

Give it cabinet posts.

 

[ditchhook]

The rice is on swamp?
Flatland is grassland, plains and tundra, not sure about ice.
Just drain the swamp.

That worked. I don't know why rice didn't work, as I assume it is on "flat land" but I read that an aquaduct can't be built on a "luxury" like silver, so maybe those special resources disqualify the "flat land". More generally, I don't like how the help files use terms like that without defining them clearly, but oh well.