Paleontology and geology
During this period, there was rapid erosion of the mountains to the west, and streams flowing eastward from the mountains carried large quantities of gravel, sand, silt, and clay. Their deposition created a broad plain with wide, shallow valleys. By the end of the Tertiary, western Kansas was covered with a veneer of sediments. The climate was wetter and milder than today, and the landscape was savanna-like with grasses on the plains and trees lining the streams and rivers.