What does an unconformity in a sequence of rock layers reveal about geologic history? A. A rock layer must have turned upside down. B. A layer of rock might have eroded. C. The layers must cover hundreds of square miles. D. The layers must have been deposited very quickly.
An unconformity, or abnormality in the layers of rocks, signifies that the process of layering was somehow interrupted. The only feasible choice for this would be (B), where erosion would have interrupted the layering process.