There are two mountain ranges within the Ozarks: the Boston Mountains of Arkansas and the St. Francois Mountains of Missouri. Buffalo Lookout, the highest point in the Ozarks, is located in the Boston Mountains. Geologically, the area is a broad dome with the exposed core in the ancient St. Francois Mountains, some of the oldest rocks in North America. The Ozarks cover nearly 47,000 square miles (120,000 km2), making it the most extensive highland region between the Appalachians and Rockies. Together with the Ouachita Mountains, the area is known as the U. S. Interior Highlands.