As little as 600,000 years ago, ancient ice sheets shaped northeast Kansas' rolling hills and rock-strewn valleys. Glaciers are rivers of ice formed when snow falls more than it melts, then slowly moves under its own weight as pressure makes lower layers flexible while upper layers remain brittle. During the ice age, two glaciers called the Independence glaciations advanced into Kansas: the first covered only the northeastern tip of the state, and the second reached south to the Kansas River and west to the Big Blue River, with ice as thick as 500 feet in some areas. As the glaciers crossed the land, they left behind tills made of clay, silt, sand, boulders, plants, and animals; in Kansas, the first glacier left a dark gray till containing the remains of a dead spruce forest, while the second deposited a brown till and erratics that are still visible today. Glacial movement also created loess, a windblown silt produced when rock was ground into fine sediment, deposited by meltwater in river valleys, and then spread by strong winds. Around the Missouri River floodplain, loess forms hills up to 350 feet high, and because it is porous and fertile, it allows roots to penetrate easily and absorb nutrients, though it is also prone to erosion. During glaciation, ice carried large boulders from South Dakota, Iowa, and Minnesota into Kansas; these erratics are often hard, erosion-resistant rocks such as pinkish Sioux quartzite. The glaciated region's natural vegetation includes forests and grasslands, with tall-to-medium prairie grasses covering much of the west and central part, forests in the east, and a transition zone between them. The fertile loess soils that supported this vegetation also made the Glacial Hills ideal for agriculture, and since settlement farmers have used the rich soil to raise corn, soybeans, hay, and cattle, along with sorghum, dairy milk, and wheat.