PRAIRIE -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. Most prairie soil was deposited during the last glacial advance that began about 110,000 years ago. The glaciers expanding southward scraped the soil, picking up material and leveling the terrain. As the glaciers retreated about 10,000 years ago, it deposited this material in the form of till. (From Wikipedia)