Natural Features

Indianapolis sits in the center of the continent in an area called the craton. While the edges of continents are more tectonically active, generating mountains and earthquakes, the stable craton is calm.

This portion of the continent was not always stable, however. From 1.5 to 1.0 billion years ago, the craton experienced a tectonically active period. The basement complex are the lowest layers of our bedrock, ranging from a depth of 3,500 to 20,000 feet. These layers are formed from igneous and metamorphic rocks. Igneous rocks form from volcanic activity and metamorphic rocks have been transformed by extreme heat and pressure, meaning this calm prairie landscape was once the site of violent tectonic shifts and volcanic eruptions.

In the next several hundred million years, sedimentary rock would form from the erosion of nearby mountains and the silt at the bottom of an inland sea. At this point, the Midwest was near the equator. Lush forests grew along the seashore and coral reefs grew in the clear water. Indiana’s famous limestone deposits were formed when the shells and skeletons of these sea creatures were deposited on the sea floor, dissolved, and reformed over hundreds of millions of years. Carbon-rich deposits like decomposing shoreline forests created Indiana’s coal deposits.

The surface landscape that we see now was created during the Ice Age, the Pleistocene Epoch that began 2.6 million years ago. A massive glacier, called the Laurentide Ice Sheet, flowed into Indiana from the Lake Huron and Lake Erie basins.

The glacier grew and melted repeatedly as the climate shifted. The flowing glacier eroded land, contributing to Central Indiana’s flat topography, and the melting glacier deposited nutrient-rich silt and soil. These glacial deposits are 100-200 feet deep in much of Marion County. Melting water from the glacier was carried away by the White River, which at times flowed underneath thousands of feet of frozen ice.