In 1869 Major John Wesley Powell, a Civil War veteran and geologist, led an expedition into the Grand Canyon to explore what he called, “the greatest exposure of the rock record on our continent.” The exposure of the “rock record” is not what draws tourists to the Grand Canyon, but it is what makes the canyon so spectacular to behold. The canyon, at its deepest is about 6,000 feet and at its widest it about 18 miles. According to the article I read, the canyon is a record of “deep” time, which means, it’s old. Powell and other geologists, knew there were three sections or “sets” of rocks that are exposed in the Grand Canyon. For example, experts say, “The three sets of rocks are differentiated on the basis of not only stratigraphic position, but also age, rock types, and overall geologic setting in which they form.”
One of the types of rocks found there are crystalline rocks which are exposed near the bottom of the canyon. Another type is sedimentary and volcanic rocks found in the coastal basins which tilted when the continents split apart. Lastly, the Paleozoic rocks at 3,000-4,000 ft. which is flat-lying sediment rocks that create the “stair step” canyon.
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There are two types of techniques that are used to date the rocks and uncover the history. First is relative dating, which can determine the order or sequences of events as they happened and second, is absolute age which determinations are numeric and identify the specific years when different things happened.
www.nature.nps.gov/geology/parks/grca/age/index.cfm
Allyson Mathis and Carl Bowman, 2006 “The Grand Canyon Age of Rocks: The Numeric Age of Rocks Exposed Within Grand Canyon, National Parks Service, U.S. Department of The