Deep underneath the soil of Indiana lies some of the hardest rock around. Limestone was deposited as sediment at the bottom of an ancient ocean some 400 million years ago. Since then the ocean has disappeared, and the sediments turned into rock, and have been buried under hundreds of feet of more recent sediment. About 100 million years ago a large meteorite struck what is now northwest Indiana, creating an impact crater about 12 kilometers, or 7.25 miles, across - the fourth largest in the United States. The impact was large enough that the central area of the crater rebounded back upward. This created high ground that was not buried as deeply by the sediments that covered the rest of the crater and the Midwest.

The large impact at Kentland caused two benefits that allow humans to use the limestone there today. First, the impact was large enough that there was a rebound effect, where the central area of the impact came back up several hundred feet. Since the rock is near the surface people can access it easily without needing to remove much soil above. Secondly, the impact fractured the rock making it much easier to remove from the ground. Today, limestone from the Kentland Quarry is used for concrete in bridges and buildings, asphalt, rail ballast, and laying the foundation for roads to local windmill farms.

Breccias are rocks that are made up of broken, angular fragments of other rocks, cemented together by finer grained material. Impact breccias form when a rock is struck by a meteorite. The "target" rocks are broken up and "glued" back together by melted rock once it solidifies. Impact breccias often contain multiple rock types because different layers of rocks are thrown out of the impact craters as "ejecta" and mix together.