Ageologist is studying an ancient stack of rock layers. most of the layers contain fossils of shallow marine creatures (clams, corals, 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 drastic 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 shallow ocean setting to a river system. b. the environment changed suddenly from a river system to a shallow ocean setting. c. a mass extinction caused a river system to form where there used to be a shallow ocean setting. d. a volcanic eruption must have occurred nearby, which scared the sea creatures away.