Student version. Students will examine maps to explore changes in population density in the United States during three decades: 1920–1930 (Post-Progressive Era), 1930–1940 (Great Depression), and 1940–1950 (World War II). They will then determine what happened during each decade that likely influenced geographic mobility. Students will also examine a map of more recent population data (for 2000–2010) to understand trends in population movement. Suggested Topics: Data visualizations Geographic mobility The Great Depression Migration Population density Post-Progressive Era World War II.