For millions of years the Gunnison River has relentlessly carved its way down through rocks nearly two billion years in age. Along 48 miles of its course in west-central Colorado it has cut one of the deepest, steepest canyons in North America. This beautiful and fascinating area was first protected as Black Canyon of the Gunnison National Monument in 1933. Since 1999 it has been known as Black Canyon of the Gunnison National Park.
Nearly two billion years ago this region was probably located along an ocean, accumulating layer after layer of sand, mud, and volcanic deposits. For hundreds of millions of years more sediments piled on, burying these beds and subjecting them to ever-increasing pressure and temperature.
Beginning around 1.7 billion years ago this once-coastal area experienced repeated tectonic collisions with island arcs over millions of years. The extreme forces further compressed and distorted these rocks, partially melting them and gradually squishing, stretching, and pulling them like taffy. The physical and chemical properties of the rocks were changed, making them harder and more dense. Over time they became gneiss and schist, choatically blended together.
Since then numerous episodes of volcanism have left their mark on the Black Canyon. Lighter-colored pegmatite dikes cut across the older metamorphic rocks, representing intrusive magma that pushed its way upward.
Repeated orogenic (mountain-building) episodes followed, and this region was gradually forced upward. Perhaps the most substantial was the Laramide Orogeny from about 70 to 40 million years ago. This period of uplift helped form the Rocky Mountains to the east and the Colorado Plateau to the west, although further uplift several million years ago increased the elevation even more.
The canyon itself began to form between ten and fifteen million years ago. Runoff and streams from the West Elk Mountains to the northeast and San Juan Mountains to the south collected in this relatively low area, forming the Gunnison River.
The Gunnison established its course in relatively soft rock that once rested upon the harder, older metamorphic layers. Once the river reached the hard gneiss and schist it was trapped in its own course and had nowhere to go but down. Over the last few million years it has chiseled away at this tough rock at the average rate of one inch every 100 years. Today the Black Canyon is over 2700 feet down at its deepest point.
The Gunnison has been able to cut through this hard rock thanks to its steep gradient and sediment load. The river drops at an average rate of 43 feet per mile, reaching as much as 240 feet per mile in the Chasm section.
This rapid descent allows the Gunnison to reach a high velocity, helping it to tear fragments of rock from its bed. The sediment load from this wear also helps to enhance its erosive action. The suspended particles act like sandpaper, further working away at the hard metamorphic rock.
Further upstream the Gunnison is slower and more serene, winding its way through more level sections of bedding.
The Gunnison is perhaps most photogenic to the east along the Blue Mesa Reservoir near the Dillon Pinnacles. The calm water and cloud-soaked mountains provide spectacular views along US-50:
The park itself also hosts a variety of interesting plants and animals including pinyon pines, junipers, oaks, great horned owls, Stellar’s jays, and peregrine falcons.
My wife and I visited this park for a couple of hours last week, and although we didn’t get a chance to see much of the wildlife we did see this mule deer near the park entrance:
East Portal Road also cuts across some private ranch land, and in the open range a number of cattle like this black angus calf were standing in the road:
On our way out this cool little store provided rocks, minerals, souvenirs, and much-needed cold drinks after our time in the sun:
Black Canyon offers some spectacular views, fascinating geology, interesting plants and animals, and a great adventure far from civilization. Although a long way from anywhere, this lesser-known national park is cool enough to be worth the trip.