These rocks were deposited millions of years ago as rivers and wind spread silt, sand, and ash across the landscape. About 500,000 years ago, ancient rivers began eroding this area, leading to the present day landscape, and modern rivers, rain, and wind continue to sculpt the land by washing away soft rock at a rate of about one inch a year. Differences in the composition of each layer reveal what the climate was like when the sediments were deposited. The oldest layers, not visible from here, represent a marine environment, while the top layers are the most recent and represent a cool, dry savannah. Together the rock layers tell a story of changing climate and ancient animal migration, adaptation, and extinction from 24 to 75 million years ago. The peaks and pinnacles of the highest rocks are part of the 28 - 30 million year old Sharps Formation, where ancient rivers deposited coarse sands and silts and ash from distant erupting volcanoes blew across the landscape; these sediments and fossils are evidence of a cool dry climate, and many of the same animals from the older Brule Formation remain. A line of gray rock called the Rockyford Ash marks the transition from the older Brule Formation to the younger Sharps Formation; 30 million years ago, this ash layer blew across the Badlands from a distant volcano and marks a specific time period that helps geologists determine the age of younger rocks above and older rocks below. In the Brule Formation, meandering rivers flowing through dry and cool shrublands and woodlands left thick sediments 30-34 million years ago, animals like oreodonts and nimravids thrived in these habitats, alligators moved away to wetter and warmer climates, and some of the colored bands in the formation are ancient soils known as paleosols.