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If you were to visit Grand Canyon National Park, you might encounter an exhibit called the “Trail of Time” and learn that researchers believe the canyon is six million years old. But not all scientists think that age is correct. Some think the canyon is 20 million years old, and a new report suggests that the canyon existed alongside dinosaurs some 70 million years ago. Melissa Block talks with Washington Post reporter Joel Achenbach, who has covered the colorful and sometimes contentious debate among researchers.