Isostatic adjustment is the process by which the Earth's crust responds to changes in surface load. When heavy ice sheets, like those from the last Ice Age, melt, the land beneath them begins to rise. This occurs because the weight that was pressing down on the crust is removed, allowing it to slowly rebound and reach a new equilibrium. This adjustment can also happen when large amounts of sediment are deposited or eroded. The crust will either sink or rise in response to these changes, maintaining balance. This phenomenon is important for understanding geological processes and the history of the Earth's surface.