If a layer of sandstone lies on top of a layer of limestone in which a 300-million-year-old fossil is found, and there are no unconformities, the layer of sandstone must be ____.

With no unconformities, the law of superposition says that in undisturbed rock layers, the layer on top is younger than the layer below it. The limestone contains a 300-million-year-old fossil, so the sandstone above it must be younger than 300 million years. The correct choice is “younger than 300 million years.”

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