Arches

General Description:
Window 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.

The Grand Staircase

The Grand Staircase is an immense sequence of sedimentary rock layers that stretch south from Bryce Canyon National Park through Zion National Park and into the Grand Canyon. In the 1870s, geologist Clarence Dutton first conceptualized this region as a huge stairway ascending out of the bottom of the Grand Canyon northward with the cliff edge of each layer forming giant steps. Dutton divided this layer cake of Earth history into five steps that he colorfully named Pink Cliffs, Grey Cliffs, White Cliffs, Vermilion Cliffs, and Chocolate Cliffs. Since then, modern geologists have further divided Dutton's steps into individual rock formations.

What makes the Grand Staircase worldly unique is that it preserves more Earth history than any other place on Earth. Geologists often liken the study of sedimentary rock layers to reading a history book--layer by layer, detailed chapter by detailed chapter. The problem is that in most places in the world, the book has been severely damaged by the rise and fall of mountains, the scouring of glaciers, etc. Usually these chapters are completely disarticulated from each other and often whole pages are just missing. Yet the Grand Staircase and the lower cliffs that comprise the Grand Canyon remain largely intact speaking to over 600 million years of continuous Earth history with only a few paragraphs missing here and there.

Unfortunately, the Grand Staircase is such a vast region of rock that no matter where you stand on its expanse, most of it will be hidden behind the curvature of Earth. Places such as Yovimpa Point and the north slope of the Kaibab Plateau are the exception where even a non-geologist can discern the individual chapters of this colossal history book--these immense steps of Dutton's Grand Staircase.

Hoodoos

General Description:
Hoodoos are tall skinny spires of rock that protrude from the bottom of arid basins and "broken" lands. Hoodoos are most commonly found in the High Plateaus region of the Colorado Plateau and in the Badlands regions of the Northern Great Plains. While hoodoos are scattered throughout these areas, nowhere in the world are they as abundant as in the northern section of Bryce Canyon National Park. In common usage, the difference between Hoodoos and pinnacles or spires is that hoodoos have a variable thickness often described as having a "totem pole-shaped body." A spire, on the other hand, has a more smooth profile or uniform thickness that tapers from the ground upward.

At Bryce Canyon, hoodoos range in size from that of an average human to heights exceeding a 10-story building. Formed in sedimentary rock, hoodoo shapes are affected by the erosional patterns of alternating hard and softer rock layers. The name given to the rock layer that forms hoodoos at Bryce Canyon is the Claron Formation. This layer has several rock types including siltstones and mudstones but is predominately limestone. Thirty to 40 million years ago this rock was "born" in an ancient lake that covered much of Western Utah. Minerals deposited within different rock types cause hoodoos to have different colors throughout their height.

Formational Process:
Hoodoos are formed by two weathering processes that continuously work together in eroding the edges of the Paunsaugunt Plateau. 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 freezes at night. When water freezes it expands by almost 10%, bit by bit prying opening cracks, making them ever wider in the same way a pothole forms in a paved road.

In addition to frost wedging, what little rain we get here also sculpts the hoodoos. Even the crystal clear air of Bryce Canyon creates slightly acidic rainwater. This weak carbonic acid can slowly dissolve limestone grain by grain. It is this process that rounds the edges of hoodoos and gives them their lumpy and bulging profiles. Where internal mudstone and siltstone layers interrupt the limestone, you can expect the rock to be more resistant to the chemical weathering because of the comparative lack of limestone. Many of the more durable hoodoos are capped with a special kind of magnesium rich limestone called dolomite. Dolomite, being fortified by the mineral magnesium, dissolves at a much slower rate, and consequently protects the weaker limestone underneath it in the same way a construction worker is protected by his/her hardhat.

Rain is also the chief source of erosion (the actual removal of the debris). In the summer, monsoon type rainstorms, travel through the Bryce Canyon region bringing short duration high intensity rain.

Preservation Message:
Unfortunately hoodoos don't last very long. The same processes that create hoodoos are equally aggressive and intent on their destruction. The average rate of erosion is calculated at 2-4 feet (.6-1.3 m) every 100 years. So it is that Bryce Canyon, as we know it, will not always be here. As the canyon continues to erode to the west it will eventually capture (perhaps 3 million years from now) the watershed of the East Fork of the Sevier River. Once this river flows through the Bryce Amphitheater it will dominate the erosional pattern, replacing hoodoos with a "V" shaped canyon and steep cliff walls typical of the weathering and erosional patterns created by flowing water. Indeed a foreshadowing of this fate can be observed in Water Canyon while hiking the Mossy Cave Trail. For over 100 years a diversion canal has been taking a portion of the East Fork of the Sevier River through this section of the park and already it's easy to see the changes the flowing water has created.

Wall and Narrows

General Description:
Walls or fins are narrow walls of rock, bound by joints or fractures on either side. As weathering and erosion opens the cracks wider and wider they form narrows or slot canyons. The wall left standing in between two slot canyons is called a fin. As fins develop, differential erosion accentuates different rock hardness leaving them with a rugose appearance.

Formational Process:
Erosion follows fractures in the sides of the Paunsaugunt Plateau called joints. Joints are common in all types of sedimentary rock and created while the rock is lithifying in the same manner that cracks form in mud as it dries. At Bryce Canyon, these joints undergo additional stresses created by the huge amounts of energy released during earthquakes along the Paunsaugunt fault and Ruby's thrust fault. While these fault lines are currently dormant many millions of years ago their activity widened and deepened the existing joints.

Snow in the winter melts a little every day and flows into joints. At night it freezes and expands breaking the rock into smaller pieces. This is called frost wedging. Bryce Canyon experiences over 200 days of freeze thaw during the year. The frequency of frost wedging in this region makes it the most important type of weathering at Bryce Canyon.