This is a wonderful case study in how something with an apparently obvious explanation can have the wildest twists and turns. All the obvious links between geology, economy and demographics lead to dead ends. Only when we turn to history does the geologic story make sense. There is a connection between geology and election returns, or rather, there are three different connections.
Demographically and economically, there appears to be nothing special about the CHAD counties. They're mostly rural and black, but in every plot of socioeconomic variables they either overlap the Control counties or continue on the same trend.
This is where the geologic story is clearest and simplest. The band of poor, black counties along the Cretaceous outcrop corresponds to the Black Belt, named for its fertile soils. Cotton production was heavily concentrated here and the black population remains concentrated.
In Georgia, the Cretaceous outcrop band is narrow and discontinuous. Topography is hilly and the soils are often rich in kaolinite, good for ceramics but poor for agriculture. Historically, agriculture and slavery were initially most prevalent in the Piedmont, where metamorphic rocks weathered to productive soils. The boundary of the Piedmont is marked by a topographic erosional scarp called the Fall Line, which often served as a locus for water power and industry. Augusta, Macon, and Columbus are on the Fall Line. Demographic maps show an early concentration of slavery north of the Fall Line.
After about 1900, chemical fertilizers made agriculture in the Coastal Plain more viable and we see a dramatic increase in Coastal Plain agriculture. The Cretaceous outcrop band is marginal for both Piedmont and Coastal Plain agriculture.
The band of poor black Democratic voting counties in South Carolina has no real connection to the Cretaceous outcrop. In fact it lies seaward of it, on Tertiary rocks. This band corresponds to what some writers call the "I-95 corridor of shame." The best explanation for this band I have found is that of Will Moredock of the Charleston City Paper, who wrote (April 11, 2012):
These are some of the poorest counties in the U.S. — majority black and culturally and economically cut off from the rest of the country and the future. They reflect the history of the region, when slaves planted and harvested vast tracts there. With the collapse of plantation agriculture, the local economy collapsed. Because it was below the fall line, it was not suitable for early industrialization. There was nothing there to attract tourism. And so the region has languished, a Third World country in our midst.
So basically, this band is a backwater. The geologic connection is tenuous at best. The fall line lies to the northwest and may have allowed a bit more prosperity in that region, although the only major Fall Line city in South Carolina is Columbia. Coastal counties benefit from trade, fishing, and, more recently, tourism. In between lies a band of poor counties, for which the only geologic explanation is they don't lie on a significant geologic structure. The principal Interstate artery along the East Coast runs the length of it, but apart from service industries at interchanges, most of the wealth simply passes through.
Before the Civil War, demographic maps show South Carolina and the Georgia Piedmont filling with slavery. Slavery moves up the Mississippi into the Delta, and moves up from the Gulf Coast of Alabama to the Black Belt, where it spreads east and west. By the time of the Civil War there is an arc of high slave population along the Black Belt and the Piedmont-Coastal Plain boundary.
The Civil War devastated the South but didn't have large immediate impact on demography or economics. What did completely redraw the economic map of the South was the boll weevil, which wiped out cotton production over large areas, and when the weevil was finally controlled, cotton production patterns shifted.
In 1900, there is still a broad arc of high black population across the South, but over the following decades blacks drifted north in the Great Migration. The present arc of poor, black, Democratic-voting counties is, in effect, an "erosional remnant" of a once wider arc with even higher black population.
Moredock, Will, 2012; South Carolina continues to neglect the I-95 corridor The State's Shame, Charleston City Paper, April 11, 2012. http://www.charlestoncitypaper.com/charleston/south-carolina-continues-to-neglect-the-i-95-corridor/Content?oid=4053180 accessed June 4, 2012.
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