The Midcontinent Plains, the Great Plains, and the grasslands are not synonymous terms.  Each describes a portion of the Continental United States defined at different times with different map resources.  the Midcontinent Plains is outlined here with the aid of the shaded relief map and the Landforms of the United States map of Erwin Raisz.  the great Plains is a outlined by the definition of Nevin Fenneman, who used geologic structure boundaries to delimit this and other of his physiographic provinces.  the grasslands are defined here from a map published in 1924 by Homer Shantz and Raphael Zon.  By the time of their work virtually all of the native grasses of the "Prairie Peninsula" area of the grasslands was lost to the plow.  Because the carbon budget of grassland soils leaves a strong impression on the soils and is used in their classification, the occurrence of Mollisols is mapped to indicate former grassland extent.

Paper on Grassland regions and modeling

By John Hudson and Dwight Brown