Geology
The beaches were formed by winds coming off Lake Michigan, which drop sand when the wind hits plants, dunes, and hills. As the lake level of Lake Michigan dropped at the end of the Ice Age, the shoreline receded, and new dunes were formed along the lakeshore. Vegetation took over the previous dunes, and eventually forests grew on top of them. There are "blowouts" along the dunes, where dead stumps were revealed after the wind blew away the sand from on top of them; the most notable such blowout is Big Blowout.
Read more about this topic: Indiana Dunes State Park