How do we reconstruct geologic history and what is the geologic time scale?
The structure of Earth’s surface and underlying rock layers and other rock bodies has been built up through billions of years of geologic history. Exposures in the Grand Canyon and other major river canyons and in mountain ranges hint at the complex assemblages that show that accumulation of ancient sedimentary layers and in fact, mountain building has occurred at one time or another in all regions of the continents. We interpret geologic history using several common sense principles for determining relative age such as: sedimentary layers in their normal sequence have older layers below and younger layers above; when features such as faults or fractures cut other features, the cross cutting features are younger; older materials may be fragmented and incorporated in younger materials; and deformational features such as folding or evidences of plastic flow are younger than the material which has been deformed. Finally, radiometric dating allows absolute age determination of many differe
