When water freezes, it swells, expands. What this means on the scale of a lake is that when the water turns to ice, it takes up more space than it did when liquid. The result is a gradual pushing up of shore sediment, a yearly “shove” by the ice as it makes room for itself. Let this happen for enough years in a row—centuries, in the case of the Great Lakes—and the result is a series of ridges along the shore of the lake. These ridges are called lake ramparts (a rampart being a mound of earth fortifying a position). A lake that has surrounded itself with lake ramparts is, thus, a walled lake.