Map of the 1860 federal census shows the density of slave population in Mississippi—more than most of the other states of the pending Confederacy.
The long, L-shaped black figure (center left of the map) extending from the northwest part of the state down into Louisiana is, of course, the Mississippi River Delta.
Although some of the Thirteenth did, indeed, own slaves, very few of the officers and men were recruited from the Delta region.
Via Dead Confederates.
13TH MISSISSIPPI INFANTRY REGIMENT 
A zoomable version of this map is here:
http://historicalcharts.noaa.gov/historicals/preview/image/CWSLAVE
And a VERY high-resolution download, with each county name and number shown, is here:
Almost two years later, neither one of these links works.
They still work for me.
And so they do now here as well. Thanks for them.