The sides of many valleys are covered by plants. The roots of the plants hold the soil in place when the rain comes, and their leaves break the force of the falling raindrops. These valleys do not erode as quickly as valleys of bare earth, but they also change over time. Water is an irresistable force. Ice and Glaciers Rocks are constantly being moved around by currents in the mantle, and they crack in response to the strain.
Rain seeps into the cracks, and when winter comes the water freezes.
Freezing water expands, and it pushes the walls of the cracks apart, making the cracks wider. Over centuries huge rocks can be split. Sometimes rocks are split off from the sides of canyons. The underneath part of the glacier may have captured rocks in the ice, and as the glacier moves, it grinds the rocks beneath it. Glaciers move all sizes of rocks, and grind some of the stones to a very fine dust called gl acial flour.