Gouged out of the face of the earth by the Colorado River. This is one of the most intricate systems of canyons, gorges, and ravines in the world. The spectacular beauty of this awesome abyss draws millions of visitors each year. But the grand canyon is more than just a tourist attraction. It is a stunning microcosm of this planets geologic history. The first geologic study of the grand canyon was led by geologist John Wesley Powell in 1869. Powell later wrote that rocks exposed in these canyon walls were an open book of geology. The Colorado River had opened the landscape, revealing one of the most complete rock records on earth, spanning almost two billion years of earth history. To a geologist, a layered sequence of sedimentary rocks is a historical record to be read like a book. Analyzing these rocks from the base to the top, were sifting through a wealth of information about conditions at the earths surface when the sediments were deposited. Rock characteristics such as mineral co