Slitherine

It's up in the top left there near the French tanks, you can compare it to normal swamps on the right hand side.

Thoughts?

What does it do? Or is it a visual overlay only?

(Is it like thick forest ie inpenetrable?)

It's totally impassable, although during snow and winter it will freeze over, allowing movement on top of and over it.
Maybe 'bog' is a better name.

Terrian variation is always welcome, and this looks pretty good. I'm sure it'll find a use.

You can use it right now. Just drop a swamp overlay on top of a lake tile.

Those ridge tiles I made earlier are actually a similar concept. Drop some plains overlays on top of coastal tiles, then change the base terrain to clear, and don't hit 'generate'.

Oh and don't forget to add your appropriate strings.