By 1860 the slave population in the United States had reached 4 million. In the 1840s, almost 300,000 slaves were transported, with Alabama and Mississippi receiving 100,000 each. During each decade between 1810 and 1860, at least 100,000 slaves were moved from their state of origin. In the final decade before the Civil War, 250,000 were moved. Historian Ira Berlin wrote:  The internal slave trade became the largest enterprise in the South outside the plantation itself, and probably the most advanced in its employment of modern transportation, finance, and publicity. The slave trade industry developed its own unique language, with terms such as "prime hands, bucks, breeding wenches, and "fancy girls" coming into common use. The expansion of the interstate slave trade contributed to the "economic revival of once depressed seaboard states" as demand accelerated the value of slaves who were subject to sale. Some traders moved their "chattels" by sea, with Norfolk to New Orleans being the most common route, but most slaves were forced to walk overland. Others were shipped downriver from such markets as Louisville on the Ohio River, and Natchez on the Mississippi. Traders created regular migration routes served by a network of slave pens, yards, and warehouses needed as temporary housing for the slaves. In addition, other vendors provided clothes, food, and supplies for slaves. As the trek advanced, some slaves were sold and new ones purchased. Berlin concluded, "In all, the slave trade, with its hubs and regional centers, its spurs and circuits, reached into every cranny of southern society. Few southerners, black or white, were untouched". New Orleans, where French colonists had established sugar cane plantations and exported sugar as the chief commodity crop, became nationally important as a slave market and port, as slaves were shipped from there upriver by steamboat to plantations on the Mississippi River; it also sold slaves who had been shipped downriver from markets such as Louisville. By 1840, it had the largest slave market in North America. It became the wealthiest and the fourth-largest city in the nation, based chiefly on the slave trade and associated businesses. The trading season was from September to May, after the harvest. One of the most famous remaining slave market buildings in the United States is the Old Slave Mart in Charleston, South Carolina. Throughout the first half of the 19th century, slaves brought into Charleston were sold at public auctions held on the north side of the Exchange and Provost building.