Aufeis: ice that grows in layers all winter
In some cold-region valleys, sheets of ice build through the winter to thicknesses of several metres and areas of square kilometres. They are not frozen lakes. They are aufeis, built layer on layer from water that keeps arriving.
The process needs a persistent water source that does not freeze at its origin — usually groundwater or a spring. As winter deepens, the channel it feeds progressively freezes, so the water is forced to the surface, where it spreads thinly and freezes. Then more arrives and spreads over that. Each cycle adds a layer, and the structure grows outward and upward through the season.
Aufeis reshapes valley floors. It can block channels, force streams to shift course, and strip vegetation. It also persists well into summer, long after ordinary river ice is gone, which makes it a meaningful influence on the local hydrograph — meltwater arrives late and gradually.
The layering is visible in cross-section where the mass has been undercut, and reading it is a little like reading tree rings, though what you are reading is a record of the winter's flow rather than of growth.