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.)