A team of geologists have conducted a survey of likely candidates to identify the oldest rock in the United States. Their ...

Parry made the discovery of the mystery rock while hunting for agates along the Yellowstone River near Greycliff.

Perry made the discovery of the mystery rock while hunting for agates along the Yellowstone River near Greycliff.

A weathered sign in the Minnesota River Valley proudly proclaims: “World’s Oldest Rock.” Erected in 1975, it marks a 3.8-billion-year-old gneiss — or so scientists thought.