An alteration in the height of the land relative to the sea; the distribution of mass within the Earth's crust is balanced by large-scale topography. These variations are not necessarily associated with changes in sea-level (eustasy), but a major event such as glaciation can affect both land and sea. The weight of ice sheets can cause a lowering in the height of the land, but a thaw at the end of a glaciation frees the land of this pressure and it rises. Continental crust behaves like a body 'floating' on the denser underlying layers. Loading of one area may cause down-warping of the crust, which is compensated by uplift elsewhere. Removal of the load causes the crust to readjust to its former state. It is a theory that the condition of approximate equilibrium in the outer part of the earth is approximately counterbalanced by a deficiency of density in the material beneath those masses, while deficiency of density in ocean waters is counterbalanced by an excess in density of the material under the oceans. This phenomenon has occurred during the Quaternary, due to the development of large ice-sheets. The enormous weight of ice has caused downwarping of the continental crust beneath. At the ice-sheet margins, there was a compensatory uplift. On melting of the ice-sheets, the crust readjusted by uplift in the areas directly underneath and downwarping at the edges. This process is continuing today, for example in northern Europe.