The colonies of the American South in the 1700s were Virginia, Maryland, Georgia, and the Carolinas. The main economic activity was agriculture, and the slave plantation system enabled large yields. Cash crops of tobacco, rice, and indigo predominated, and pine forests were also productive for the building and furniture industries. Cotton was grown in the 18th century but would not become a major crop until the 19th century. The long growing season due to the warmth of the South and the fertile soil made for favorable conditions for farming. Historians estimate that in the 18th century around six million Africans were imported into the colonies as slaves who worked the farms and plantations of the South. Much of what was produced was shipped to English ports.