Montrose - Daily Press

A mere spec on the geologic timeline

The sun had risen several hours earlier, its long slanting rays slowly penetrating the dark canyon depths and highlighting the narrow, winding river far below. Ominous shadows were at once swallowed up by the warm spring sun and the sheer gray walls and pinnacles of the canyon once again reappeared from the black abyss. Another day had dawned at the Black Canyon.

Both prehistoric and historic (Ute) peoples knew of this mammoth gorge. Although these first residents were superstitious of the canyon’s depths, they judiciously used the resources along the rims for centuries. Modern man’s interaction with the Black Canyon of the Gunnison has primarily taken place in the past century, representing a tiny pinprick in the history of a natural landmark that holds valuable clues to the earth’s earliest beginnings.

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The Black Canyon of the Gunnison has often been described as a gigantic crack that seemingly reaches into the bowels of the earth. Slicing through layer upon layer of rock, the gorge is revealed much like cutting through the layers of a cake—only in this instance the layers have been distorted by some of the most powerful forces of Mother Nature.

The earliest beginnings of the Black Canyon started some 60 million years ago during a period of great mountain building and volcanic activity in the region known today as western Colorado. What resulted from this violent upheaval in the earth’s crust was the “Gunnison Uplift,” an extensive elevated area where drainages were forced into narrower and more restricted channels. After the volcanic activity died down, about 2 million years ago, downcutting began. A river (the modern Gunnison) started across a terrain of volcanic rocks; once its course was established, it followed along the fold of the Gunnison Uplift and the irreversible process of erosion began the slow, continuous wearing away of rock by water, sediment and time.

The rate of downcutting has been approximately one foot per 1,000 years.

Over the millennia, the river cut a course through some of the oldest rocks on earth, the hard, crystalline formations once sand and mud on an ancient sea floor. Smaller tributaries which flowed into the canyon from both sides could not keep pace with the river, resulting in steep canyon walls. The re-routing of the Upper Colorado River System far back in time (from Unaweep Canyon to the north) also had an impact on the Black Canyon. The new course flowed through soft Mancos shale, commonly known as adobe. Consequently, the lower Gunnison, North Fork and Uncompahgre Rivers lowered their beds more rapidly than the upper Gunnison, steepening its overall gradient.

Most of the Precambrian (dating from origin of the earth to about 570 million years ago, representing nearly 90 percent of geological time) rocks, such as granite, gneiss, schists, found in the bottom of the Black Canyon are more than a billion years old. The upper sedimentary layers of the canyon are comprised of shale and sandstone.

The geological processes which formed the canyon left behind an even more valuable resource ... rocks suitable for the manufacture of stone tools. The upper sedimentary layers of the canyon contained material such as chert, chalcedony and quartzite, ideal for the production of projectile points and stone knives as well as tools for processing larger game.

What’s new with

the old ...

Geologists are now using terms like “tectonic evolution,” “incision history,” “deformation,” and “river profiling” to further define the processes that formed the nearly 53 mile long Black Canyon gorge  that lies just east of Montrose.

At a meeting of the Geological Society of America (Rocky Mountain Section) held in May of 2006, researchers from the University of New Mexico at Albuquerque presented an update and summary on the “Incision History of the Black Canyon of the Gunnison.”

Much of the recent work of researcher Dr. Karl Karlstrom has focused on the ancient drainages that once fed the Gunnison, including the abandoned Shinn-Bostwick “river” to the south and Grizzley Creek on the north side. These drainages, Karlstrom noted, hold the key to better understanding the relationship between surface uplift and climate in creating the landscape we see today.

Intermittent layers of “fill gravel” have also been found from the confluence of the North Fork of the Gunnison River to the highest part of the Black Canyon rim. What further adds to the puzzle is that the gravels found at the confluence of the north rim and Grizzley Creek are a mix of both volcanics (youngest rocks) and Precambrian (oldest rocks) fragments, indicating different incision (downcutting) rates have occurred in different parts of the canyon at different times.

What does all this mean for the average visitor to the Black Canyon of the Gunnison National Park?

The true beauty and mystique of the canyon will remain the same, and the seasons will still come and go, but in geologic terms, researchers are giving us a clearer picture of what really happened eons ago in the carving of the Black Canyon of the Gunnison.

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