I was just trying to clarify what people mean when they say "The Delta" because an earlier description wasn't on point.

No, of course the music doesn't stop at the bluffline. But the alluvial plain is a special place because of its geography and fertility, and that influenced its demographics, and that influenced the music.   It is 5 million acres of rich flat land that required massive amounts of human labor to farm. Most of that labor was African American. If you look at old aerials of it, you can see that about every 10 acres was a house where a tenant farmer lived, because it took a family for about every 10 acres. Never mind men to work the mills, clear the timber, and build the levees.

There are lots of small towns, and a few cities including Greenville and Memphis and Greenwood, and back in the day there were lots of juke joints and houses that sponsored music on weekend nights, and there developed a culture of musicians gathering locally, and as they got more professional, traveling around to other towns. This is the scene described by David Honeyboy Edwards in his book.

Certainly similar things were going on on the Arkansas and Louisiana side and in spots up in hill towns, but I think there was a dense concentration of people and talent here that of course Lomax thought was the central cradle of the music.