What was the political and economic transformation that began in the United States in the late 18th century?

In the late 18th century and early 19th century, the U.S. began to undergo what is generally called the "Market Revolution," in which new modes of transportation facilitated and expedited the transfer of goods to markets. Transportation networks in the eastern and mid-western (then the western) U.S. developed, including canals, railroads, and steamboats. These means of transportation, such as the Erie Canal in New York (which connected Albany to Lake Erie), connected the eastern and western parts of the U.S. Raw materials from farms in the west could easily be shipped to production centers in the east. This meant that the eastern and western parts of the country were developing capitalist economies, and they were also becoming more connected politically. 

However, the south largely did not develop these types of transportation networks (except steamboats), and the south did not industrialize except in a select few cities such as New Orleans but stayed committed to slavery. For example, the south had far less mileage of railroad tracks than the north and west. As a result, the south was dependent on finished goods from the north or imported goods from Europe, and the south started to become more and more disconnected from other parts of the country politically as well.