A geologist is studying an ancient stack of rock layers. Most of the layers contain fossils of shallow marine creatures (clams, corals, etc.), suggesting the layers formed in a shallow ocean environment. But at the top of the stack, there is a layer that could only have been deposited by a river. What does this change in rock layers tell the geologist about Earth's history in the area where these layers formed? (Assume the layers have not changed position since they formed.) A. The environment changed suddenly from a river system to a shallow ocean setting. B. A volcanic eruption must have occurred nearby, which scared the sea creatures away. C. The environment changed suddenly from a shallow ocean setting to a river system. D. A mass extinction caused a river system to form where there used to be a shallow ocean setting.

The presence of the fossils of shallow oceanic creatures like corals points to the fact that the environment was a shallow ocean at some point. There is also a presence of a layer on top of that which points to a river system.

The fact that the oceanic fossils are beneath the river system layer means that the ocean came first and the river system layer came second. The environment must therefore have been a shallow ocean setting that changed into a river system.