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HOW DO GLACIERS FORM VALLEYS?

We read about valleys that were shaped/formed by glaciers. Since glaciers are just ice, and ice is just water, how can it be that glaciers can cause the formation of alleys? Frequently, the text will inform us that the glaciers "grind" the walls of the valley. Let's find out how that can be.

Rub an ice cube across the painted surface of a board. Note that the ice is not hard enough to abrade the surface.

Now press the ice cube into a dish of sand. When the ice has melted a bit, place the dish, with the ice cube still in it, in the freezer. After the ice cube has refrozen, remove the cube from the dish, and rub it across the board again.

Looks like it made a few scratches, doesn't it? By freezing some grains of sand into the ice cube, we caused it to have an abrasive surface.

The constant melting and re-freezing of glaciers is only a small part of what causes them to move along the surface of the land. Gravity, coupled with their tremendous weight, is mainly what causes them to move. While moving, they pick up various sediments, which are subsequently frozen into, and become a part of, the glaciers. (Glaciers are not "just" ice!) Glaciers are rocks! Glaciers are minerals!

Although glaciers have some qualities of igneous rock (because they began as liquid), and some qualities of sedimentary rock (because they pick up some of whatever
surfaces they touch), they are actually classified as metamorphic rocks!

This will blow your minds even more: glaciers are plastic on the bottom, which means they are subject to being formed by the surfaces they touch. The top parts of glaciers are called "fracture zones," because they are not plastic; they crack and break because they are brittle. The plastic bottom parts of glaciers are the parts that move, and the brittle, top "fracture zones" are carried along by the plastic part!

Sometimes, glaciers pick up rocks that are bigger than cars! They carry them along for awhile, and, if a substantial portion of the lower ice melts, the glacier may leave that huge rock behind. We call those huge rocks that we see in places where they don't match the surrounding rocks, "erratics." What happens when a rock that is carried by a glacier rubs against other rocks and soil? The rock in the glacier (and the glacier itself) will wear (grind) away the areas through which the glacier moves. Valleys created by glaciers are always U-shaped. As the glaciers move along, they deepen and widen the valleys, by taking rocks and soils with them.

 
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