These are manmade, but they were created in the 1970s, 80s, and 90s for waterfowl. they are mounded earth, brush, and trees that serve as islands for geese to lay eggs and raise their young relatively free from coyotes and other predators. As the previous poster said, they were a joint project between Ducks Unlimited and the federal government that has been a resounding success. In fact, most of the water features on the Devil's Garden have been built for waterfowl habitat. It's one of the most productive areas for breeding on the continent.

The mounds at the Grass Lake rest area on us-97 are the same things.