Mountains, Ozark << OH zahrk, >> are a range of hills extending from southern Illinois across Missouri and into Arkansas and Oklahoma. The Ozarks rise from 1,500 to 2,300 feet (457 to 701 meters) above sea level. Their highest peaks are the Boston Mountains of Arkansas. The Ozarks are part of the Ozark region, which also includes areas of flatland. The region covers about 40,000 square miles (100,000 square kilometers). The hills are covered with timber. There are many caves in the areas where the hills are made of limestone. The Ozarks of southeastern Missouri contain the most important lead mines in the United States. Mines in the Ozarks also produce iron and barite. The raising of beef cattle is the most important agricultural activity over most of the Ozark region. Farmers raise dairy cattle on the Springfield Plateau in southwest Missouri. Crops grown in the Ozark region include apples, grapes, peaches, and wheat. Tourism is a major source of income in many parts of the region. See also Missouri (Land regions) .