The formation of the North American Prairies started with the upwelling of the Rocky Mountains. The mountains created a rain shadow that killed most of the trees.The parent material of most prairie soil was deposited during the last glacial advance that began about 110,000 years ago. The glaciers expanding southward scraped the landscape, picking up geologic material and leveling the terrain. As the glaciers retreated about 10,000 years ago, it deposited this material in the form of till. Wind based loess deposits also form an important parent material for prairie soils.
Tall grass Prairie evolved over tens of thousands of years with the disturbances of grazing and fire. Native ungulates such as bison, elk, and white-tailed deer, roamed the expansive, diverse, plentiful grassland before European colonization of the Americas. For 10,000-20,000 years native people used fire annually as a tool to assist in hunting, transportation and safety. Evidence of ignition sources of fire in the tallgrass prairie are overwhelmingly human as opposed to lightning. Humans, and grazing animals, were active participants in the process of prairie formation and the establishment of the diversity of graminoid and forbs species.
Fire has the effect on prairies of removing trees, clearing dead plant matter, and changing the availability of certain nutrients in the soil from the ash produced. Fire kills the vascular tissue of trees, but not prairie, as up to 75% (depending on the species) of the total plant biomass is below the soil surface and will re-grow from its deep (up to 6 feet) roots. Without disturbance, trees will encroach on a grassland, cast shade, which suppresses the understory. Prairie and widely spaced oak trees evolved to coexist in the oak savanna ecosystem.