Deep and narrow, with its floor often obscured by dark shadows, the mighty river-carved chasm at times appears to be a bottomless abyss. Some other canyons may be narrower, deeper, or steeper-walled. But few anywhere in the world can rival the Black Canyon of the Gunnison for the combination of all three qualities: in places this gloomy, sheer-walled gash in the face of the earth is deeper than it is wide.
In the most spectacular segment of the gorge, which has been preserved as a national monument, the depth ranges from 1,730 to 2,700 feet (525 to 825 meters). At its narrowest point, the canyon is only 1,100 feet (335 meters) wide at its rim and tapers down to a mere 40 feet (12 meters) across at the bottom. Hemmed in by somber gray, nearly vertical and even overhanging walls, the depths of the chasm are lost in shadow much of the time—hence the name Black Canyon.
The canyon extends for 53 miles (85 kilometers) about midway along the course of the Gunnison River in west-em Colorado. In places the cliffs that bound it are a truly dizzying spectacle. One of them, the Painted Wall, drops almost straight down for 2,250 feet (685 meters) from the canyon’s rim to the river’s edge. Just as Captain John Gunnison, who explored the region in 1853, wisely skirted the canyon that is the river’s most impressive feature, most visitors today are content to view the spectacle from scenic overlooks on the rim. Only the hardiest hikers and most experienced rock climbers venture down into its depths, where fallen boulders as big as houses litter the canyon floor.
The rocks exposed in the canyon walls are mainly extraordinarily hard, erosion-resistant schist, gneiss, and granite that were formed more than 1 billion years ago. But while the rocks are very ancient, the canyon itself is quite young, geologically speaking. The raging waters of the Gunnison River carved this stupendous chasm over the course of the past 2 million years or so.
How did the river manage to carve a canyon in such erosion-resistant rock? The explanation is based on events that occurred far back in time. Many millions of years ago, layers of sedimentary rock were deposited on the flat, eroded surface of the ancient bedrock. The sedimentary rock, in turn, was partially eroded away and then was covered by a layer of soft volcanic rock.
Flowing over these easily eroded sedimentary and volcanic rocks, the Gunnison River firmly established its course across the landscape and cut its way downward to the basement rock. Since the river by then was too firmly entrenched to change its course, it had no alternative but to continue cutting its way downward into the bedrock.
It was mainly the river’s large volume of flow and very steep gradient through the canyon that enabled it to carve so deep a trench through such resistant rock. (The Gunnison drops an average of 95 feet per mile, or 18 meters per kilometer, through the monument.) Armed with rock debris, the raging river was a powerful cutting tool capable of carving the steep-walled chasm and lowering its bed more rapidly than other forces could widen the canyon.
Today dams upstream have slowed the river’s flow and moderated the effects of seasonal floods. Even so, it is doubtful that the Gunnison will ever be a gentle stream. The rate of erosion may have been slowed, but it has not been stopped. Sarah writes for the Prague Guide and lives near Prague Castle.