The great weight of glaciers causes the ice to grind away at nearby rocks to form a fine sediment called called rock flour, and large rocks from the valley and canyon walls as it moves.  This experiment actually simulates what happens when a glacier melts and finally deposits the rock flour (chalk) and large boulders (gravel). When the glacier melts it drops boulders into a deposit called a moraine and glacial rock flour gives mountain lakes their milky color. 

Here is a fun experiment to learn more about moraines. 

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