This map shows how unevenly the slave population was distributed across the South. The distribution of slaves did have some correlation with the support for secession. In the large upland area stretching southwest from the border with Pennsylvania, which had relatively few slaves, support for the Union was significant. The western counties of Virginia, for example, refused to secede when the rest of Virginia did. West Virginia was admitted to the Union in 1863.
Source: Henry S. Graham Washington, 1861. Library of Congress, https://www.loc.gov/item/99447026/.