Hoodoos Of Bryce Canyon
The mystical shapes inspire imagination and intrigue. It appears impossible that the destructive forces of water carved these fragile landforms. Instead many believe the hoodoos of Bryce Canyon were formed by wind. This is a mistaken idea. Wind is an effective form of erosion for many locations. However, for Bryce Canyon wind has little effect on the creation and destruction of the various shapes.
Hoodoos formed over thousands of years by the same processes that form the features of surrounding parks. Water, ice (at varying intervals) and gravity are the forces that form Bryce Canyon. These three erosive forces coupled with the differential erosion of the four rock types of the Claron Formation produced a different morphology than that of other parks. 10-15 million years ago the Paunsaugunt Plateau was caught and uplifted by the Colorado Plateau. Breaks called joints formed in the plateau during the uplift. Joints allowed water to flow into the rock. As water flowed through joints erosion widened them into rivulets and gullies. Over time, deep slot canyons formed in the sides of the plateau.
Bryce Canyon receives an average rainfall of 10 inches a year in the valley and approximately 19 inches a year on the plateau. Majority of the precipitation falls in mid to late summer. It comes in the form of monsoons, usually in the afternoon. These thunderstorms can be fierce, dropping an inch or two of rain in under an hour. Hail storms often accompany heavy rains in the region. Most of the rainfall is not absorb by the thin layer of soil covering the rocks. Because soils at Bryce Canyon are very dry, much of the water runs off the surface. Only the top inch, or so,of soil absorbs rainfall before it starts to run off. When this happens at Bryce Canyon flash floods are the result.
At Bryce Canyon water in the form of ice is the most efficient form of erosion for breaking rock into smaller pieces. The Paunsaugunt Plateau receives approximately 100 inches of snowfall a year. It also experiences about 200 days of freeze/ thaw. Which means that everyday a small amount of snow melts and runs into the joints and freezes at night. When water freezes it expands to form an ice wedge in the joint widening the space. As the ice wedge grows by more water leaking into the joint and freezing it will finally break the rock.
This is called frost wedging. Small pebbles and large Volkswagen bus sized boulders commonly fall from the sides of existing hoodoos and the sides of the Paunsaugunt Plateau by frost wedging and gravity. The smaller pieces are washed away by the monsoons and snow melt. Boulders explode into cobble sized pieces when they impact the ground. The resulting debris is washed down slope by snowmelt, flashfloods and gravity.
Rock type is another factor in the creation of the bizarre shapes of hoodoos. The hoodoos at Bryce Canyon are carved in the Claron Formation. Limestone, siltstone,dolomite and mudstonemake up the four different rock types that form the Claron Formation. Each rock type erodes at different rates. The rock's resistance to erosion is what causes the undulating shapes of the hoodoos.
Dolomite, limestone and siltstone are very hard and form the protective caprock on most of the spires. Frost wedging is the erosional force that breaks apart the harder rocks. Mudstone is the softest rock in a hoodoo and is easily identified because it forms the narrowest portion of the pinnacles. As mudstone moistens it erodes easily and will run down the sides forming a stucco or protective coating. Every time it rains the stucco is renewed.
Eolian or wind forces erode at slow rates. If wind does not erode the stucco layer fast enough it will renew before eolian erosion affects the rock. For this reason wind has little to no affect on hoodoo formation or destruction.
Another sign that wind is not responsible for hoodoo formation is the absence of a dominant wind direction pattern carved into the plateau. If eolian forces were responsible for erosion in the area hoodoos would erode parallel with the prevailing wind direction.
At Bryce Canyon National Park hoodoos do not align with the prevailing air currents. Hoodoos protrude from the sides of the plateau in lines that follow joint patterns caused by faults that uplifted the area. Some of the walls and fins meet at almost a 90-degree angle. Wind erosion would not form patterns like this.
After a heavy rain, soil in the region dries out and forms a hard crust. Wind erosion requires loose particles to be transported with in the air column and impacted into a surface for erosion to take place. The particles need to be free and not attached to a crust. Because of the crust there is not a large supply of loose particles to be used for wind erosion in the area. Vegetation also keeps loose grains in place by their roots and by slowing the gusts of wind as it blows through the branches.
Erosional scours paralleling the hillslopes are carved on the slot canyon walls. They are common in many places within the park. As hikers descend into Wall Street on the Navajo Loop a close inspectionof the canyon walls will show diagonal erosive marks cutting through horizontal bedding planes. These scour marks show the erosion point of historical hillslopes. If wind erosion were responsible for the formation of walls, fins and hoodoos the old scour marks would have been eroded away along with the hillslope.
Hoodoo surfaces would be smoother and more round if wind erosion were responsible for their creation. Instead many hoodoos have sharp edges and jagged tops like the skyline of 'The Silent City'. Angular pinnacles, stuccoed sides, boxy erosional patterns and scour marks are some of the more
obvious tells of water erosion.
While visiting Bryce Canyon National Park look for signs of wind and water erosion. It is suprizing how visible the numerous signs of water erosion are, when you know what to look for.
Source National Park Service, Geologic Resources Division photos by DesertUSA
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