Level 3 earth spell "Raise Land", costs 5 mana --> turns it into hills.  These are build-able.  If you want, you can then cast "Lower Land" (also L3 Earth, 5 mana) to turn it into plains, but this isn't necessary if you just want to place a city building there.

Note that "drained" swampland is not necessarily fertile; you may still have to cast Birth of Summer/Bloom of Twilight to allow you to found a city there (but the above is not necessary just to extend into the tile from an existing city.)

Note that there's also a swamp wildland - you need to clear the wildland to make this area fertile/buildable.

Thanks guys, no it was for regular city expansion, I just like building towards other cities or resources or whatever, and an outpost does not continue a city. If you raze the outpost you get a blank tile. I was hesitant to use raise or lower land because I don't know what it does to the resources close to the spell location.

The situation I was working with was one tile of swamp between me and my horsies, so I built and outpost, raised it, and then built two 1 tile buildings to link my horsies. That's why I razed the outpost.

So the raise and lower spells are nice, but my question being does it erase the horses, gold mines or whatever that are right beside where you cast it?

Yes, though if you build cities like I do, then you plop it in the middle of 3 resources and snake your way to all of them, or forest and rivers for example, I rarely have enough buildings left to build to encase an outpost.

Thanks so much for the answer, this makes that spell much more valuable to me, I was thinking it was like raise/lower in Alpha Centauri that does wipe out resources that are close to where your terraformer is.

Well, I have heard tell and seen the odd screenshot where people raise mountains around their territory with only key choke points for entry and exit.

Raise Land cannot create mountains, and Lower Land cannot create water.  The best you can do is turn the tiles into Hill terrain, which will slow down invaders, but not stop them entirely.

The only way to create a mountain in the current version of the unmodded game is via use of Curgen's Volcano, which would be a bit extravagant for creating chokepoints.