Rupert Cornwell writes for The Independent.

One way and another, surveyors have left their mark on American history. George Washington started his career as one. Then came Charles Mason and Jeremiah Dixon, two from Britain who in the 1760s used their skills to settle a boundary dispute between the then colonies of Pennsylvania and Maryland.

Whether or not the word "Dixie" derives from Jeremiah's surname is unclear. (According to another theory, it originates with the "Dix" on the back of $10 bills in New Orleans.) But the physical line he helped to demarcate, and its later unofficial extension west along the Ohio river, came to symbolise the great divide between the North and the South, between the slave and non-slave states that fought the American Civil War.

Even now, along today's Maryland/Pennsylvania line, you can still see some of the old mile markers of the Mason-Dixon line, great 500lb slabs of limestone shipped from England. But no longer does this 233-mile border symbolise America's ancestral cultural divide. That line is sliding gently but inexorably southwards, testament to a shift in US society that could play a critical role in this year's election.

In fact, the unofficial western extension still holds good; the mighty Ohio remains a North/South frontier. If Washington DC fell to the Confederate armies, Abraham Lincoln reputedly said, he would set up a new capital in Ohio, which then was the third most populous state in the Union. But head south across the river from Cincinnati and you're in another world – or, more exactly, Kentucky. The first town you hit is Florence, or as the sign on the water tower that dominates the skyline has it, "Florence, Y'all"...