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Our transcription: The first geologic study of the Grand Canyon was led by Geologist John Wesley Powell in 1869. Powell later wrote that the rocks exposed in these canyon wall were an open book to geology. He recognized that the Colorado River had laid open the landscape to reveal one of the most complete rock records anywhere on Earth, one that spans almost two billion years of Earth's history. To geologists a layered sequence of sedimentary rock like this is a historical record that can be read like a book. As we analyze these rocks from the base of the stack to the top, we're sifting through a wealth of information about the conditions of the Earth's surface at the time the sediments were deposited. Rock characteristics such as mineral composition, grain size and shape, structures within the rock, and even rock color tell us a great deal about the climate, vegetation patterns, position of the shoreline, and the topography of the Earth's surface in the geologic past. The challenge of sedimentary geology is to interpret these clues in the rocks, but in order to do so, we must first understand how sediment is formed, and how it's transformed into solid rock.
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