American school children learn that slavery caused the American Civil War. Other historians argue that this explanation is simplistic, and there were other contributing factors. Whether it was the sole cause or not, it's impossible to deny that slavery was a huge factor. Now, cartographer Bill Rankin has shown just how sharp the divide between North and South really was in a series of illuminating heat maps that visualize slavery in the United States during this era.
Rankin's maps start in 1790, fourteen years after America established its independence, and the first census year. That year was peak slavery for the North, where there were slightly more than 40,000 slaves, accounting for around 2% of the total population. Comparatively, 654,121 people—or about 34% of the population—were enslaved in 1790 in the South. That percentage remained consistent until 1860, when nearly 4 million people were enslaved in Southern states, compared to literally zero in the North.
According to Rankin, mapping slavery in America presents numerous problems. For one, slave distribution throughout America was never uniform, and even in the South, there were certain states and counties that were more or less likely to be slaveholding. In 1860, for example, Delaware and Maryland had fewer slaves than the Mississippi belt, where the percentage of slaves approached 95% of the total population. "Should a county with 10,000 people and 1,000 slaves appear the same as one that has 100 people and 10 slaves?" Rankin asks. So instead of shading large geopolitical areas with a single color based upon the number of slaves inside it, Rankin took a pixelated approach. He divided the country into 250-square-mile cells and colored them individually, in the hopes of "[refocusing] the visual argument of the maps—away from arbitrary jurisdictions and toward human beings."
After visualizing slavery by decade, Rankin put together a map visualizing Peak Slavery across the country, where each dot represents the 10-year period in which any given area owned the most slaves. What it shows is that slavery wasn't petering out in the South when the Civil War was fought. In fact, with the exception of a few areas, Rankin says "slavery in the South was only headed in one direction [in 1860]: up." Yet by 1870, the North and South had the exact same number of slaves: zero.
The Civil War may have been a bloody, traumatic ordeal with a legacy that still shapes the country today, but Rankin's maps make one thing crystal clear: Without the Civil War, slavery wasn't just going to fizzle out.