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rock
A rock from our yard. What caused all of the curved markings?

Answer:

The curved markings are called chatter marks. They happen when a rock that breaks with a conchoidal fracture hits another rock. Think about what you see when a small rock hits a glass windshield. You get a circular break in the glass. That is a conchoidal fracture. Each of the circles on this rock indicates a hard collision with another rock.
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