The 12 contiguous states of the North Central Region constitute one of the most productive agricultural areas in the world. Before the region was settled, vegetation consisted primarily of forest in the east, grasses in the west, and an intermingling of grass and forest in the central portions. Although the region contains a high proportion of productive soils, there are great differences within the region in soils, as well as in climate, vegetation, and geology. The short-grass soils in the western areas are higher in pH, lighter in color, and thinner than the tail-grass soils in the central and west-central areas, which have thick, dark surface horizons. The more strongly leached soils of the eastern humid forested areas are more acid and have thin, dark surface (A) horizons and light-colored subsurface horizons, which are mixed when cultivated. Approximately 75% of all soils in the region is derived from glacial till and loess. A large proportion of the deposits of glacial origin was derived locally from underlying bedrock or from material laid down during previous glaciations. Most of these deposits range from a few to tens of feet thick, but occasionally are several hundred feet thick. Loess deposits deeper than 4 feet cover large areas of Illinois, Iowa, Missouri, Kansas, and Nebraska. There also are extensive areas where shallow loess deposits cover the land surface.