In 1673 Frenchmen Louis Joliet and Father Jacques Marquette explored the areas of the Mississippi and Illinois Rivers, and their voyages led to French claims on the region until 1763, when France ceded the land to Great Britain by the Treaty of Paris. During the American Revolution, George Rogers Clark and his army won the Illinois Territory for the Commonwealth of Virginia. In 1784 it became part of the Northwest Territory, and on December 3, 1818, Illinois entered the Union as the twenty-first state. U.S. Route 24 enters Illinois at Quincy, site of the sixth debate between Abraham Lincoln and Stephen A. Douglas in 1858, then extends east toward the Illinois River across the Military Tract, an area used as bounty land for veterans of the War of 1812. It passes near Dickson Mounds, a burial site of the Mississippian culture circa 1100 A.D. Route 24 crosses the Illinois River at Peoria, a center of agricultural equipment manufacturing, and near the site of Fort Creve Coeur, erected by the French explorer Sieur de LaSalle in 1680. It bisects Eureka, site of the college attended by Ronald Reagan, the 40th President of the United States and the only one born in Illinois. The route continues east through some of the richest farmland in the nation, where between 1840 and 1890 huge herds of cattle were raised for sale to eastern markets and where corn and beans are the main crops today. Route 24 exits Illinois east of Watseka near the Old Hubbard Trace, a fur trade route linking the Wabash River area with the settlement of Chicago in the north. Along its approximate 260 mile length, Route 24 passes through eleven of Illinois' 102 counties and seven of its county seats.