Arches

Common Name (preferred): Arches
Geologic Name: Windows
Size Range: 3-60 ft. (1-19m) in diameter
Formation Name: Claron Formation
Rock Age: Most common in sedimentary rock. At Bryce often seen in the Limestone of the Claron Formation.
Famous Examples: Natural Bridge & Wall of Windows

 

Double window formation

nps

General Description:
Windows or arches are natural holes that form along cracks and weak spots in thin walls of rock called "fins." By convention these holes must be at least 3 feet in diameter in two perpendicular directions to earn the name arch or window. An imprecise distinction is often made between bridges and arches in terms of the processes that form them. It's important to remember that gravity is the key factor in either case. Nevertheless, the distinction is that bridges are carved by flowing water, whereas arches can be carved by everything else except flowing water. Indeed, in very few circumstances is it possible to say that flowing water had zero contribution in the development of one of these natural holes. Therefore, geologists often prefer the term window to collectively describe any large hole in a rock. At Bryce Canyon most of our windows are carved by frost wedging.

 

Formational Process:
Windows start as narrow fractures that run through the rock. At Bryce Canyon these fractures include expansion joints and secondary earthquake fractures. These fractures intersect each other at right angles creating a checkerboard-like pattern of weak cracks in the rock.

 

 

Weathering and erosion carve through these cracks steadily widening them, opening up slot canyons, leaving behind walls or fins in-between. The primary weathering force at Bryce Canyon is frost wedging. Here we experience over 200 freeze/thaw cycles each year. In the winter, melting snow in the form of water, seeps into the cracks and refreezes at night. When water freezes it expands by almost 10%, bit by bit, forcing the cracks wider and wider in the same way a pothole forms in a road.

At the same time this process is converting ridges into fins, it is also forming windows along the perpendicular fractures within individual fins. Once a window becomes too large to support its own roof it will collapse leaving one leg of the window standing detached - thus creating a hoodoo.

Preservation Message:
Many people are surprised to learn that the National Park Service makes no effort to protect natural windows from collapsing. In simplified terms our agency is not overly concerned with natural products as much as we are perpetuating natural processes. This is why park rangers in Glacier National Park don't stop male grizzly bears from killing baby bears even though Grizzlies are an endangered species. Infanticide is a natural process that ensures that only the offspring of the most intelligent, powerful and protective female bears will survive and thus add to the gene pool. Such is the case with beautiful windows. The natural process is weathering and erosion, and besides, if it weren't for collapsing arches, we wouldn't have hoodoos.

When and where to see at Bryce:
There are probably hundreds of windows within the boundaries of Bryce Canyon National Park. The vast majority are either small or well hidden and usually go unnoticed. Changing sun angles through the course of the day and with seasonal variation can make some windows almost invisible. Every arch has its own best viewing time and season. The largest and most readily accessible is the Natural Bridge located about half-way down the southern scenic drive. The Peek-a-Boo Loop Trail and the Mossy Cave Trail are target-rich hikes for those who love to see natural windows.

 

 

Further Reading:
DeCourten, Frank. 1994. Shadows of Time: the Geology of Bryce Canyon National Park. Bryce Canyon Natural History Association, Bryce Canyon, Utah

Ritter, Dale F. Kochel, R. Craig. Miller, Jerry R. 1995. Process Geomorphology 3rd edition. Wm. C. Brown Publishers. 365-66.

Sprinkel, Chidsey, & Andersons (eds) 2000. Geology of Utah's Parks and Monuments. Utah Geological Association. Bryce Canyon Natural History Association, Bryce Canyon, Utah

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