As the last ice age was ending, the winds blew fine ground soil into dunes along what is now the Missouri River. Dunes made of this ‘loess’ soil (German for “loose”) piled up over 200 feet in some parts of Iowa over thousands of years. They can be found elsewhere in the world but Iowa’s are special: They’re second to China’s in height.
The Loess Hills of western Iowa stretch 200 miles across the state into Missouri, and the Loess Hills Scenic Byway is an easy way to meander through them.
See these strange hills up close at Broken Kettle Grasslands, a Nature Conservancy preserve about 200 miles northwest of Des Moines. There are no trails at the preserve, but visits can be arranged. For a more tourist-accessible spot, you can try the 15 miles of trails and campground at Waubonsie State Park.