Natural History

The inland sea

The natural forces that produced the 21st-century landscape explored by Natural Bloomington tours were set in motion during the geologic era known as the Mississippian Period – 323 million to 354 million years ago – when most of what is now the United States was covered by a shallow, inland sea. The state was near the equator at the time, and as the sea evolved and retreated over tens of millions of years, the mineral deposits it left behind laid the foundations for today's landforms.

The forested hills and valleys included in the Full Circle, East to Brown County and Old Growth tours, for example, evolved from a large Mississippian delta system that left behind a resistant type of bedrock known as the Borden Group. Consisting of siltstones, shales, fine sandstones and isolated carbonates, the Borden belt stretches from Benton County in the north-central part of the state – through Morgan, Monroe, Brown and Jackson counties – to Floyd County on the Ohio River.

The gently rolling, sinkhole-riddled landscape explored in the Karst Country Tour, on the other hand, sits atop limestone, formed when Mississippian-era snails and other marine animals with shells made primarily of calcium carbonate died and settled to the sea floor. In addition to sinkholes, the limestone belt is characterized by underground rivers and streams and extends from Southeast Owen County to Harrison County on the Ohio.

A particularly unique, valuable form called Salem Limestone, Bedford Limestone or, simply, Indiana Limestone, is found only in a 10-by-35-mile strip of land in Monroe and Lawrence Counties. Coveted for its light color, fine grain, versatility and workability, Indiana Limestone is a major architectural feature in the Indiana Statehouse in Indianapolis; City Hall and Tribune Tower in Chicago; Empire State Building in New York City; the National Cathedral, Pentagon and Department of Commerce in Washington D.C.; courthouses and government buildings from New York to Sacramento; and academic buildings on college campuses across the country, most significantly IU-Bloomington.

The Great Ice Age

The Great Ice Age, which lasted more than a million years, was the next earth-moving phenomenon to leave its mark on the Bloomington region’s 21st-century terrain. Toward its end, roughly 16,000 years ago, continental ice glaciers still covered most of Indiana.

Driven by the pressure of their own weights and the force of gravity, the glaciers ebbed and flowed like giant, frozen rivers, moving the earth, breaking down rocks and grinding out hills. For centuries, they moved about a foot a day.

When they retreated for the final time about 10,000 years ago, the massive ice sheets left two types of landforms in their wakes: the flat plains of the state’s northern and central regions and the hills of the south.

The glaciers covered the northern and central parts of Indiana, leaving behind dunes, hills, rivers, lakes and flatlands when they melted. The southern part of the state was left uncovered but not untouched. Erosion from floods and melted ice carved the landscape in this part of the state.

Most of Monroe County west of Bloomington lies in the Mitchell Plain, a physiographic region that extends from eastern Owen County through western Monroe and south to the Ohio River at Harrison County. Melting glacier ice eroded and shaped the limestone bedrock into a geologic formation known as karst topography, with rolling hills, sinkholes and underground caves and streams.

East of the Mitchell Plain lies the Norman Upland, a serpentine landform characterized by rugged hills and valleys that extends from Southern Morgan County through Monroe and Brown counties south to the Ohio River at Floyd County.


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