 Complete the following phrase:  From 1956 to 1978, USAC Championship Car seasons featured the top teams and drivers in U.S. open-wheel racing. Until 1971, the Championship contained road courses, ovals, dirt courses, and, on occasion, a hill climb. Thereafter, the schedule consisted mainly of paved ovals. In 1979, the majority of car owners left USAC to race in the Indy Car World Series, sanctioned by the Sports Car Club of America and CART. This led to a decline in the number of events in the schedule, and by the 1984-85 season, the Championship comprised only one race, the Indianapolis 500. The era of USAC Championship Cars concluded with the formation of the Indy Racing League IRL in 1995, which was sanctioned by USAC until June 1997 when the IRL assumed officiating duties after the controversial finish of the 