The Appalachian mountain range known for having some of North America's oldest exposed rock formations is the Adirondacks.

The Adirondack Mountains, located in northeastern New York, feature ancient metamorphic rocks that are over a billion years old. These rocks were formed during significant geological activity, including tectonic shifts and volcanic activity. The Adirondacks are distinct within the Appalachian system, as they are a dome-shaped uplift rather than a typical mountain range formed by folding. Their geological history contributes to the rich biodiversity and unique ecosystems found in the region.