Archaean Metamorphic Rocks
The Archean rocks in Colorado are the oldest. They were metamorphosed about 2.7 billion years before present. That means that the original rocks were even older. Unfortunately, there aren’t very many of them, only about 40 acres in extreme northwest Colorado along Beaver Creek.
The image below shows the 2.7 billion-year-old metamorphic rocks (black and white layers)of the Archean Owiyukuts Complex overlain unconformably by the basal, reddish sandstones and conglomerates of the 25,000 foot-thick, Neoproterozoic Uinta Group that were deposited about 775 million years ago.