Chapter 1: Precambrian

The Precambrian accounts for Earth's history 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! The story would reveal a time of harsh and drastic changes in the Earth and show little to no sign of life.

It is difficult for geologists to interpret what Colorado may have looked like during this time, because most of this region's Precambrian-age rocks have been highly altered by extreme heat and pressures.

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 the 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.

 
example of gneiss, a rock with dark and light stripes or bands
Gneiss - notice the intense folding of alternating light and dark bands.

NPS photo

Gneiss

The metamorphic rock that dominates the walls of the Black Canyon is called gneiss (pronounced "nice"), and is blended with schist, another rock that normally has flat or elongated crystals. You might spot the intense folding of the alternating light and dark bands in this adjacent photo. These rocks were once buried deep below the Earth's surface where they encountered extreme heat and pressure.

 
thick pink stripes of pegmatite in a large cliff face, fog
Pegmatite - some of the best examples in the park are intruded in the Painted Wall (above).

NPS photo

Pegmatite

This granite-like igneous rock formed as hot magma forced its way into cracks. It cooled slowly, allowing large crystals to form. The pegmatite is loaded with shiny muscovite (mus'-ko-vite) mica and large crystals of a pinkish mineral called potassium feldspar.

 
The overlooks along the South Rim Drive offer excellent panoramic views of the canyon, and a chance for close examination of the Precambrian rocks that form the canyon walls. You might especially enjoy viewing the rocks along the Oak Flat Trail or the walk out to Rock Point.
 

Last updated: October 22, 2019

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