A dialect is a given style and pattern of word use and pronunciation among a specific part of a population.  In the pre-Civil War period (also called the Antebellum period), American society was static.  That is to say, people didn't move around much, and rarely traveled very far from home.

So the dialects from say, Alabama, or Missouri, or Georgia or Virginia were much more distinct than they are today.  The language had not been blended with the dialects of outsiders, or soldiers, or a more mobile people like we have today, so they were also much more recognizable at the time.  You would know a Georgia accent when you heard it.