John Wesley Powell called the Grand Canyon of the Colorado the best geological section on the continent in 1868. The geological section he referred to is a vertical cross section of exposed Grand Canyon rock layers, visible at Pima Point, that has inspired and motivated geologists since his time. Nowhere on earth is the world's geologic past laid open better than here. Grand Canyon geology was one of several factors that led Major Powell to undertake his daring boat run of the Colorado River in 1869. As he explored the Canyon's rocks, he wrote of climbing from granite up over rust-colored sandstones and greenish-yellow shales until the men and boats were lost in the black depths below, the dashing river seemed a rippling brook, and more canyon still rose above than lay below; all around him, he said, were interesting geological records in an open book he could read as he ran. Along Grand Canyon's entire 277-mile length, scenes like this at Pima Point remain an open book.