Answer:
Slavery in the southern US began to expand in the Virginia Colony with the tobacco plantations in the Chesapeake area. It expanded southwards with time, first to the Carolinas, were slaves were imported to work in Rice Plantations, and, finally the Georgia colony.
However, cotton began to replace tobacco, rice and sugar in the South, because it was more profitable, and suited better the soil and climate of the region. With the cultivation of cotton, slavery became widespread in the US. The majority of slaves were "imported" from the end of the 17th century to the first decades of the 18th century.
Cotton plantations made then, slavery institutions of the antebellum South widespread, strong and entrenched. The economics of the South practically depended on slavery, just like the economics of the Roman Empire a millenium ago.