Analyzing rock formations yields clues that help establish ages and events in Earth’s 4.6-billion-year past. The branch of geology concerned with this is called stratigraphy. Two important laws, or principles, of stratigraphy are the law of original horizontality and the law of lateral continuity. According to the law of original horizontality, sediments are laid down in horizontal, or nearly horizontal, layers. The law of lateral continuity states that these layers are deposited over large areas. You might think of the way snow falls to help illustrate this. If drifts haven’t formed, snow will accumulate in a layer across your yard. And, as that happens, it’s not just snowing on your yard. Snow is falling on a large area, probably your whole city or town.

Which sentence was included in the passage to illustrate the law of original horizontality?