Any reason why Madison/Limestone has a much higher percentage than other areas there?
They're on the north bank of the Tennessee River and are pretty flat (east Madison County is not but the western half of the county, which houses Huntsville, is). This makes for lots of irrigable land with good soil. This allowed for large, productive plantation estates in ways that rockier, more rugged terrain can not.
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It's not as pronounced as the eastern parts of the Appalachians but you can see that the counties in question are still pretty rugged and not amenable to large productive estates like the lowlands further south are.
Sol is correct that the counties in question are appalachian and didn't the same social, economic, and demographic influences that plantations had on other areas of the state.
