When mountains are first formed, they are tall and jagged like the Rocky Mountains on the west coast of North America. Over time (millions of years) mountains become old mountains like the Appalachian Mountains on the east coast of Canada and the United States. When mountains are old, they are rounded and much lower. What happens in the meantime is that lots of rock gets worn away due to erosion. Rain, freeze/thaw cycle, wind and running water cause the big mountains to crumble a little bit at a time.

Eventually most of the broken bits of the rock end up in the streams & rivers that flow down from the mountains. These little bits of rock & sand are called sediments. When the water slows down enough, these sediments settle to the bottom of the lake or oceans they run into. Over many years, layers of different rock bits settle at the bottom of lakes and oceans.

Think of each layer as a page in a book. One piece of paper is not heavy. But a stack of telephone books is very heavy & would squish anything that was underneath. Over time the layers of sand and mud at the bottom of lakes & oceans turned into rocks. These are called sedimentary rocks.

You may have a difficult time imagining something solid like rocks wearing down over time - but everything does. If you take a look in the lint trap of your dryer, you will see that your clothes are being worn away as they tumble in the dryer. In fact if there is enough lint - you will see how these bits have been laid down into layers - just like sediments at the bottom of the lake. You will see layers of different colors because the clothes you dried were different - just like you will see different layers of rocks in sedimentary rocks. What is even more interesting is that if you scrunch up the lint a bit like in the picture here, you can see the layers of lint bending - just like the layers of rock are bent. Look carefully at the rocks in road cuts and you sill see layers of rocks that have been folded just like the lint in your dryer. Neat eh?!

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