Stratigraphy

The study of stratigraphy was at this point in its infancy, but it was already understood that different layers of rocks had been formed during different periods.

On Earth, we chart the history of waterways by looking at sediment that is deposited over time. We call that stratigraphy, the idea that water transports sediment and you can measure the changes on Earth by understanding the way that sediment piles up. That's what we've done here—but it's Mars.