The formation of badlands is a result of two processes: deposition and erosion. The process of deposition describes the accumulation, over time, of layers of mineral material. Different environments such as seas, rivers, or tropical zones, deposit different sorts of clays, silts, and sand. For instance, the badlands formations in Badlands National Park, South Dakota, United States underwent a 47-million year period of deposition which spanned three major geologic periods (the Cretaceous Period, the Late Eocene, and the Oligocene Epochs), resulting in clear, distinct layers of sediment which serve as a dramatic display of the law of superposition. Once the deposited sediments have solidified, the sedimentary material becomes subject to erosion. It is sometimes erroneously taught that badlands erode at a steady rate of about one inch or 25 millimetres per year. In actuality, the precise processes by which the erosion responses take place vary depending on the precise interbedding of the sedimentary material. In 2010, researchers at Badlands National Park initiated a three-year project to learn more about the actual erosion rate of the specific badlands found in that park.