In this topic you will learn about how wind, water, and ice reshape the world.

Most of Earth's surface is not perfectly flat. Some places are higher, and some places are lower. Gravity is always pulling things from high places down to low places. The downhill movement of Earth material caused by gravity is called mass wasting. How steep a slope is will affect mass wasting. It can happen slowly, particle by particle, over years, or it can happen suddenly after a heavy rain or an earthquake.

The sediment is dropped off at the bottom of the hill or at places where the hill becomes less steep. The dropping off of sediment is called deposition.

Erosion and deposition work together. Sediment is picked up in one place by wind or water. Then it is dropped off somewhere else. As this happens, the shape of the land changes. Mountains wear down. Valleys widen, fill up with rock and soil, and become plains.

Wind can blow sediment. Windblown sand can build up into a dune. The dunes change shape and appear to drift forward in the wind direction.

Flowing water carries loose particles of rock. The pieces of rock carried by moving water act like tiny drills. The pieces of rock erode the sides and bottom of the river. When the water slows down, some of the particles are dropped off.

A huge sheet of ice and snow that moves slowly over the land is called a glacier. Some glaciers form in valleys high up in the mountains. Others form near the poles. Glaciers form when more snow falls in the winter than melts in the summer. When the ice gets about 100 meters thick it can move. The whole sheet of ice moves downhill. When a glacier moves over the ground, pieces of rock may freeze into the ice. Huge chunks of rock may be picked up and carried long distances.

Glaciers eventually reach places where it is warm, and they melt. When the ice melts, the rocks that were frozen into it fall to the ground. A jumble of many sizes of sediment deposited by a glacier is called a till.

A deposit of many sizes of sediment in front of or along the sides of a glacier is called a moraine. As the glacier melts away, the moraine is left as mounds or long ridges. Glaciers also form lakes. The glacier scrapes a huge hole in the ground and the hole fills up with meltwater when the glacier melts. Chunks of glacial ice can also get buried in till. When ice chunks melt, the till collapses, forming a bowl-like hole in the ground filled with the meltwater.