Sand production

The water in HePing River is very filthy with sand and mud in it.  The teacher took a tank to fill the water in the river.  Everyone looked at it and said, “Wow, How dirty it is!”   The teachter told us that sand was weathered from rocks.  Rivers scrub rocks and turn them into smaller pieces.  Rivers rise and fall, and make the rocks on the bank smaller and smaller.  After years, they become fine sand.