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By carefully digging, we have found that each trash pit shows a sequence of layers.
Look at the many photographs of the Grand Canyon in your textbook.
Note that you can follow the layers all along the walls of the canyon, and you can find the same layers on both sides of the canyon.
He concluded that process that are currently operating on the Earth must be the same processes that operated in the past.
This principle is often stated as "the present is the key to the past".
The Grand Canyon is particularly good for this because different sedimentary rocks have different colors.