A third type of rock, Sedimentary rocks provide the most detailed picture of past environments.
Modern desert dune fields such as those at Death Valley (left) are composed of fine, well-sorted sand. If we could bulldoze a dune in half, we would see large scale cross beds. From the Principle of Uniformitarianism, was can say that the Navajo Sandstone in Zion Park, Utah (photo at right), which has the same features, was formed in a desert dune field. [Left Image courtesy of the NPS]