Black Canyon of the Gunnison National Park is a United States National Park located in western Colorado, and managed by the National Park Service. There are two entrances to the park; the more-developed south rim entrance is located 15 miles (24 km) east of Montrose, while the north rim entrance is located 11 miles (18 km) south of Crawford and is closed in the winter. The park contains 12 miles (19 km) of the 48-mile (77 km) long canyon of the Gunnison river. The national park itself contains the deepest and most dramatic section of the canyon, but the canyon continues upstream into the Curecanti National Recreation Area and downstream into the Gunnison Gorge National Conservation Area.
Geology
The Gunnison River drops an average of 34 feet per mile (5 m/km) through the entire canyon, making the 5th steepest mountain descents in North America. In comparison, the Colorado River through the Grand Canyon drops an average of 7.5 feet per mile (1.4 m/km). The greatest descent of the Gunnison River occurs in the park at Chasm View dropping 240 feet per mile (45 m/km). The Black Canyon is so named on account of its steepness which makes it difficult for sunlight to penetrate very far down the canyon. As a result, the canyon walls are most often in shadow, causing the rocky walls to appear black. At its narrowest point the canyon is only 40 feet (12 m) across at the river.
The extreme steepness and depth of the Black Canyon formed as the result of several geologic processes acting together. The Gunnison River is primarily responsible for carving the canyon, though several other geologic events had to occur in order to form the canyon as it is seen today.
Precambrian
The Precambrian gneiss and schist that makes up the majority of the steep walls of the black canyon formed 1.7 billion years ago during a metamorphic period brought on by the collision of ancient volcanic island arcs with the southern end of what is present-day Wyoming. The lighter-colored pegmatite dikes that can be seen crosscutting the basement rocks formed during this period as well.
Cretaceous - Tertiary
The entire area underwent uplift during the Laramide orogeny between 70 and 40 million years ago which was also part of the Gunnison Uplift. This raised the Precambrian gneisses and schists that make up the canyon walls. During the Tertiary from 26 to 35 million years ago large episodes of volcanism occurred in the area immediately surrounding the present day Black Canyon. The West Elk Mountains, La Sal Mountains, Henry Mountains, and Abajo Mountains all contributed to burying the area in several thousand feet of volcanic ash and debris.
The modern Gunnison River set its course 15 million years ago as the run-off from the nearby La Sal and West Elk Mountains and the Sawatch Range began carving through the relatively soft volcanic deposits.
Quaternary
With the Gunnison River’s course set, a broad uplift in the area from 2 to 3 million years ago caused the river to cut down through the softer volcanic deposits. Eventually the river reached the Precambrian rocks of the Gunnison Uplift. Since the river was unable to change its course, it began scouring through the extremely hard metamorphic rocks of the Gunnison Uplift. The river’s flow was much larger than currently, with much higher levels of turbidity. As a result the river dug down through the Precambrian gneiss and schist at the rate of 1-inch (25 mm) every 100 years. The extreme hardness of the metamorphic rock along with the relative quickness with which the river carved through them created the steep walls that can be seen today.
A number of feeder canyons running into the Black Canyon slope in the wrong direction for water to flow into the canyon. It is believed that less-entrenched streams in the region have shifted to a more north-flowing drainage pattern in response to a change in the tilt of the surrounding terrain. The west-flowing Gunnison, however, was essentially trapped in the hard Precambrian rock of the Black Canyon and could not change its course.
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