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Glaciers are rivers of ice. They slowly melt but each year more snow falls on them, so they last for a very long time - thousands of years! Gravity makes them move by sliding down hill but they only move about as quickly as your fingernails grow.

Glaciers are so big and heavy, they can carry things that fall onto them, like rocks. Sometimes rocks can be carried for miles but it takes hundreds of years for this to happen! As the bottom of the glacier scrapes along the ground, it slowly digs it up. This is called erosion. Over hundreds or thousands of years, glaciers can dig out so much rock and soil that it makes a valley. Glaciers are quite wide and flat, so the valleys they make usually have a 'U' shape.

cheyenne-rouse-climbers-crevasse-emmons-glacier-mt-rainier-wa.jpgGlaciers are strong and very slippery because they are made of ice. When they move along the valley, they have to bend around corners and over bumps, and they get cracks in them. These cracks are called crevaces. People can walk on glaciers but it is very dangerous because if you fall into a crevace, it can be very hard or impossible to get back out again.