When a plant or animal is buried quickly, it is protected from decaying and has a better chance of becoming a fossil. Groundwater moving through the silica-rich volcanic ash and other sediments can dissolve the silica, or quartz. When this silica-rich water soaked into the trees, organic compounds in the wood were dissolved and replaced by very small crystals of quartz. In some cases, the quartz crystals are so small that much of the internal structure of the trees is preserved, including the growth rings. This process of quartz replacing wood is called petrifaction.
Geologic processes continue to shape the badlands.
Yearly precipitation in the badlands averages 15 inches. Rain, though infrequent at times, usually comes in heavy, erosive downpours. Water running down slope forms gullies, while some soaks into clay-rich rocks and soils. The added weight of water sometimes causes portions of hill sides to break loose and flow downhill. Lightning strikes and prairie fires can ignite coal beds, which then may burn for many years.