Hundreds or even a thousand meters deep and continents wide, the scale and majesty of glaciers is difficult to comprehend. They command 10% of the world's land area and 70% of its freshwater. Their birth and death alter global sea levels to the tens or hundreds of meters. The ice within them always flows, behaving as a ductile solid below 60 meters; above this, the brittle ice cracks into crevasses up to 10 meters wide. When glaciers advance, they grind landscapes, leaving behind unmistakable calling cards in the form of carved mountains, valleys, and lakes. A few of these landforms are discussed below.