Neatorama reader Eugene Chekanov sent us this amazing photo of an "ice cave," basically the inside of an ice pressure ridge:
Pressure ridge is basically a zone where ice either collides or retreats. In my case, it was collision of two ice masses - one is being pushed from the river and the other being pushed in the opposite direction from the lake. When these two ice masses collided they pushed each other up, therefore, creating a dome, about 1.5 m high. The most striking thing is that - being close to the shore, still in the beginning of solidifying - the ice froze between the rocks. Once it started to lift it took all the rocks with it creating the ice cave with the walls that have the rocks embedded in them!
This particular ice cave is big enough to fit people!
http://www.eugenechekanov.com/icescapes2.html - Thanks Eugene!