Figure 8.21:
The influence of terrain on agriculture. The spatial relationship of commercial grain farming and flat terrain appears in the American Midwest in about 1960. Flat terrain is defined as any land with a slope of 3° or less. Commercial grain farming is completely mechanized, and flat land permits more efficient machine operation. The result is this striking correlation between a type of agriculture and a type of terrain.
(Source: After Hidore, 1963.)