The following picture is that of a gravity map of the United States.What do the dark grey regions mean in terms of the density of the material below? 
Definitions:
John J. Pershing
A senior United States Army officer who commanded the American Expeditionary Forces on the Western Front in World War I.
African Americans
Refers to citizens or residents of the United States who have ancestry from any of the black racial groups of Africa.
Menial Labor
Work that requires minimal skill and is often characterized by low pay and status.
Integrated Units
Refers to military or other organizational units that include members without regard to race, sex, religion, or national origin, focusing on unity and teamwork.
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