Indiana has two principal types of terrain: slightly rolling land in the northern half of the state and rugged hills in the southern, extending to the Ohio River. The highest point in the state, a hill in Franklin Township (Wayne County), is 1,257 ft (383 m) above sea level; the lowest point, on the Ohio River, is 320 ft (98 m). The richest soil is in the north-central region, where the retreating glacier during the last Ice Age enriched the soil, scooped out lakes, and cut passageways for rivers.
Four-fifths of the state's land is drained by the Wabash River, which flows westward across the north-central region and turns southward to empty into the Ohio, and by its tributaries, the White, Eel, Mississinewa, and Tippecanoe rivers. The northern region is drained by the Maumee River, which flows into Lake Erie at Toledo, Ohio, and by the Kankakee River, which joins the Illinois River in Illinois. In the southwest, the two White River forks empty into the Wabash, and in the southeast, the Whitewater River flows into the Ohio.
In addition to Lake Michigan on the northwestern border, there are more than 400 lakes in the northern part of the state. The largest lakes include Wawasee, Maxinkuckee, Freeman, and Shafer. There are mineral springs at French Lick and West Baden in Orange County, and two large caves at Wyandotte and Marengo in adjoining Crawford County.
The underlying rock strata found in Indiana were formed from sediments deposited during the Paleozoic era, when the land was submerged. About 400 million years ago, the first uplift of land, the Cincinnati arch, divided the Indiana region into two basins, a small one in the north and a large one in the southwest. The land was steadily elevated and at one time formed a lush swamp, which dried up some 200 million years ago when the climate cooled. During the Ice Ages, about five-sixths of the land lay under ice some 2,000 ft (600 m) thick. The retreat of the glacier more than 10,000 years ago left excellent topsoil and drainage conditions in Indiana.