It seems like you're asking about the largest land feature in the Southern United States and what plains it's associated with. However, the question is a bit ambiguous.

Here's why:

* "Southern States" is broad: This could refer to the entire region, including the Coastal Plain, Piedmont, Appalachian Mountains, and even parts of the Great Plains.

* "Biggest land feature" is open to interpretation: This could mean the largest in area, highest elevation, or most prominent geographically.

To give you a better answer, I need more context. Could you clarify what you mean by:

* "Southern States": Do you mean the states that are part of the U.S. South, or a more specific area within the South?

* "Biggest land feature": What criteria are you using to define "biggest"?

Once you provide more information, I can give you a more specific and accurate answer.