Illinois Natural Areas Inventory defines natural areas as
tracts of land or water that contain relatively undisturbed natural communities containing plants and animals that were present prior to European settlement.These areas must also have the potential for protection and management. Natural areas are further categorized based on their attributes. These include
Grade B forests include those that are lightly disturbed, or have been minimally disturbed in the past. Forests that were selectively logged in the past might qualify as grade B natural areas. Grades C through E might be applied to second growth forests, with the degree of disturbance increasing with the increase in letter grade.