Big Belt Mountains

This picture was taken of the Big Belt Mountains from I-15 near Spring Creek Recreation Road, Spring Creek, Montana.  More than a billion years ago, during the Precambrian Era, an inland seaway deposited the shales and sands, which over time became the red and green mudstones shown in the photo.  These mudstones (Spokane Shale), originally laid as horizontal layers, became tilted when the floor of the Pacific Ocean slowly collided with the Western Margin of North America.  The red and green mudstones are more prominent when photo is enlarged.