When it is cold for more than about half a year, the snow that falls in winter may not fully melt in summer. As a result, snow will build on snow. Snow is made of very delicate crystals of ice. When snow piles up, the little spikes that make up snowflakes get crushed, and the snow compacts until it is solid ice.
If the snow and ice build up enough, say to a depth of many tens or hundreds of metres, the weight of the ice causes it to begin to flow. This happens very slowly. It is not noticeable on flat land, but on the steep slopes of a mountain valley, the ice may flow and slide down the mountain by many metres a year.
As the ice moves across rock it changes the shape of the valley it is in. This is how it happens. Ice is so cold it is sticky (as you will know if you get something out of a freezer). The ice sticks to rock of the valley floor, and as the ice moves down the valley, so it pulls pieces of rock away.
The rock trapped in the bottom of the ice is scraped over the valley floor, and it acts like sandpaper, wearing away more rock. But in this case the scraping produces fine rock flour.
This is why the bottom of the glacier contains a mixture of ice, rock flour, stones and boulders.
The wearing action of the glacier is almost entirely on the bottom. As a result, the bottom gets worn away while the sides do not, and over many thousands of years this makes the floor fairly flat and broad. The result is a valley shape that looks like a letter U, and glaciated valleys are known as U-shaped valleys.
