The Colorado River has been forming the Grand Canyon in Arizona for at least 6 million years. During this time, the river has slowly caused erosion of the rock. As the rock eroded, soil eroded as well, and plant populations have had to adapt to live in increasingly rocky conditions. The river continues to deepen and erode the canyon at a rate of a few milimeters per year. What geologic principle is this an example of? gradualism geographism uniformitarianism catastrophism
