Indiana Glaciers

About 4 million years ago, the late Cenozoic ice age began. During an ice age, cyclic climate changes occur such that snowfalls do not completely melt the following summer. Ice sheets advance and retreat on temperate areas at intervals of about every 100,000 years. These shorter events are known as glaciations. The Illinoian glaciation peaked ~220,000 years ago and its maximum southern extent is indicated by the blue line at right. The Wisconsinan glaciation (red) peaked about 20,000 years ago.

These events had a major impact on Indiana geology. Glacial sediments buried much of the karst present in the older limestones north of these glacial boundaries. Today, this buried karst supplies an important groundwater source. Glacial boundaries and their meltwaters also defined the current course of all the major rivers in Indiana.

For various reasons, the regions we now call the Mitchell Plateau and Crawford Upland were mostly spared glaciation. Where this exposed bedrock coincides with the Blue River and Sanders group limestones shown in the Stratigraphy slide, karst terrain displays its most extensive development.