File:Yellowstone River valley badlands near Miles City Montana 003 - 2013-07-03.jpg

English: Sandstone cliffs and hoodoos near Miles City, Montana.

From 105 to about 60 million years ago, North America between the Rocky Mountains and Appalachians, the Deep South, and parts of Hudson Bay were under water. This area, known as the Western Interior Seaway, was 2,500 feet (760 m) deep. Vast quantities of plankton died here over the millennia, their tiny calcium shells forming a huge plate of limestone over most of this area. Much later, in Montana, volcanism occurred -- laying down a thin layer of very hard basalt over the soft limestone.

Over time, the basalt weathered away or cracked, exposing the soft limestone beneath. Most of the limestone weathered away. But in a few places, a "cap" of basalt was left -- shielding the limestone below. This forms pillars of limestone capped with a thin slab of harder rock -- a structure known as a "hoodoo".

The main part of a hoodoo is soft limestone, and it is not unusual for wind to erode holes in it, or for birds to peck out a nesting area there. When the wind blows, hoodoos can make a soft moaning sound. Native Americans believed these places to be the home of spirits, while early white settlers thought them haunted.