Dolomite- Chicago's Bedrock

A good example of how the four spheres of physical geography work together to shape the earth’s surface can be seen in the formation of the bedrock foundation upon which Chicago rests. Dolomite, a magnesium-rich limestone that began to form 400 million years ago, makes up the majority of this foundation. At this time, the area Chicago now inhabits was at the bottom of a tropical, inland sea (the atmosphere and hydrosphere). Over the course of millions of years, the skeletons and shells of countless sea creatures (the biosphere) settled to the bottom of this ocean and eventually formed the area’s limestone bedrock (the lithosphere).

This bedrock is only visible at the surface in a very few places in the region. This is because throughout most of the region, this bedrock was covered by the debris left behind by the glaciers when they receded for the last time. In some places, this covering is 400 feet thick. In others, the bedrock is exposed.

The bedrock, however, has little effect on the natural physical features of the land above it. Most of these are due to the effects of the glaciers. What it has had a major effect upon are the unnatural physical features of the landscape. In particular, the presence of the bedrock has provided anchorage for the skyscrapers which are the hallmark of Chicago’s architecture. When these buildings are constructed, their foundations are anchored in the bedrock and this allows for the incredible heights to be reached.