North Carolina Grade 8 End-of-Grade (EOG) Science Practice Test

Rocks become rounded and smooth when they are carried by moving water because they are worn down by abrasion as they rub against other rocks and the riverbed during transport. This gradual wearing away is erosion, and when water is the agent, it includes the smoothing that happens as rocks tumble and grind together. Fossilization is about preservation of organisms, and weathering by ice or abrasion by wind describe other ways rocks can be worn but not the water-driven rounding seen in flowing water. The water transport specifically brings about the rounded, polished shapes through continual grinding.