There are three distinct topographical regions in Ohio. They are the foothills of the Allegheny Mountains in the eastern half of the state; the Erie lakeshore, extending for nearly three-fourths of the northern boundary; and the central plains in the western half of the state.
The Allegheny Plateau in eastern Ohio comprises of rugged hills and steep valleys that move away gradually as the terrain sweeps westward toward the central plains. The highest point in the state is Campbell Hill at a height of 1,549 ft/472 m, in Logan County about 50 mi (80 km) northwest of Columbus.
The Erie lakeshore, a band of level lowland that runs across the state to the northwestern corner on the Michigan boundary, is characterized by sandy beaches. The central plains extend to the western boundary with Indiana. The lowest point of the state is located on the bands of the Ohio River in the southwest, where the altitude drops to 455 ft (139 m) above sea level.
There are more than 2000 lakes in Ohio and most of them are situated in the east, and nearly all are reservoirs backed up by river dams. Pymatuning Reservoir is the largest reservoir located on the Pennsylvania border and has an area of 14,650 acres (5,929 ha). The largest lake is the Grand Lake (St. Mary's), located near the western border, covering 12,500 acres (5,059 ha).
Ohio has two drainage basins separated by a low ridge extending from the northeast corner to about the middle of the western border with Indiana. At the north of the ridge, more than one-third of Ohio’s area is drained by the Maumee, Portage, Sandusky, Cuyahoga, and Grand Rivers into Lake Erie. South of the ridge, the remaining two-thirds of the state is drained mainly by the Muskingum, Hocking, Raccoon, Scioto, Little Miami, and Miami rivers into the Ohio River, which winds for about 450 mi (725 km) along the eastern and southern borders.
The oldest limestone rocks are found in the Cincinnati anticline, a ridge of sedimentary rock layers about 3,000 ft (900 m) thick that extends from north to south in west-central Ohio. Millions of years ago, glaciers covering two-thirds of the state leveled the western region into plains and deposited fertile limestone topsoil. Gradually with time, the melting ice formed a vast lake, which overflowed southward into the channels that became the Ohio River. Perhaps 15,000 years ago, during the last Ice Age, the glacial waters ran off and reduced Lake Erie to its present size. Marks of glacial movements can be seen in limestone rocks in Glacier Grooves State Park on Kelley's Island.