At the Vault History blog, Rebecca Onion posted a really cool map of the United States in 1861 (shown above), which uses data from the 1860 census to determine the percentage of enslaved people per county in the southern states. Onion explains that:
The map, which shades counties based on the percentage of total inhabitants who were enslaved, shows what a range there was in levels of Southern enslavement. Some counties, the map explains, “appear comparatively light … this arises from the preponderance of whites and free blacks in the large towns in these counties.” The population of Orleans Parish, La., in one example, was 8.9 percent enslaved. Places that were rural but were located in mountainous areas devoid of plantations were similarly light-shaded: The people of Harlan County, Ky., were 2.3 percent enslaved.
Meanwhile, a dark belt of counties bordering the Mississippi River held more than 70 percent of their residents in slavery, with Tensas Parish, La., at 90.8 percent and Washington County, Miss., at 92.3 percent.