Anchor-ice

Anchor-ice, or GROUND-ICE, a kind of ice which forms upon the beds of rivers, or shallow brackish seas. It only begins to form when the temperature of the atmosphere falls to within 10° F., and it does not adhere strongly to the bottom until zero is reached. It is somewhat porous or sponge-like in appearance, and when it rises to the surface, it frequently brings with it the stones or boulders to which it is attached. It is where the flow of the water is most interrupted and tumultuous that this kind of ice forms most readily; it does not appear to form in perfectly still water, however clear. See ICE.