So anyway... I had found a nice place. Running water that's unfrozen all year, a volcano, and a perfectly square valley that amused me too much to NOT use. I was capping off my volcano, because that's what I do, with large amounts of water. After draining the water away, the last level of water, instead of evaporating, all froze quite suddenly.
The picture's after I decided to put my metalworking down there anyway, and had a little magma spill, but all around it is ice, with, and this is important, running water not a little ways off (off-screen). I tried digging down through it, that's the blocks of ice down at the bottom. Claimed one of my miners actually, so I decided to leave it be after that.
Is the mountain in a different temperature biome? I chose temperate deliberately after losing a fortress to frozen water.
Or... Am I just that cool?
[EDIT]
All the ice melted. Apparently the volcano's in a cold biome and the valley's in a temperate one. On the plus side I could make my freezing trap drop the frozen invaders into magma afterwards. Hmm...