Precambrian accounts for the history of the Earth from its very beginning up until about 540 million years ago. If we condensed all of Earth's history into a 1000 page book, the Precambrian would fill pages 1 through 880 -- most of the book! The story would reveal a time of harsh and drastic changes in the Earth and show very little to no sign of life.
Because most of Colorado's Precambrain-age rocks have been highly altered by extreme heat and pressures, it is difficult for geologists to interpret what this region may have looked like during this time. So we usually find ways to describe the types of Precambrian rocks, and note where and how they may have been formed.
Precambrian rocks are often called basement rocks since they are usually buried deep beneath the surface. They only become exposed under special circumstances where the overlying younger rocks have been stripped away. Most exposures of these ancient rocks are found in the cores of mountain ranges or in deeply eroded canyons like the Black Canyon.
In Black Canyon of the Gunnison National Park, the Gunnison River cuts through Precambrian rock nearly 2 billion years old! Most of these rocks are metamorphic and show evidence of exposure to extreme pressures and temperatures. Some of the rocks are igneous and formed from magma that pushed its way up into cracks in the Earth's crust, where it cooled and crystallized.