The Geology of Cedar Breaks National Monument, Utah: How an Amphitheater of Hoodoos Formed

Cedar Breaks National Monument covers about 6,155 acres in south-central Utah, at an average elevation of about 11,000 feet (3,353 m). Its centerpiece is a vast erosional amphitheater cut into the western rim of the Markagunt Plateau at 10,400 feet (3,170 m) — the plateau that holds the highest ground in southwestern Utah. A few miles north, basalt-capped Brian Head Peak rises to 11,315 feet (3,449 m). Glacial and periglacial processes have also reshaped this eroded landscape.

This page draws on the National Park Service's Geodiversity Atlas, built from the NPS Geologic Resources Inventory. Geodiversity means the full range of a park's geologic and soil resources — rocks, minerals, sediments, fossils, landforms — and the processes that shape them.

A step on the Grand Staircase

In the 1870s, Major John Wesley Powell began geological surveys of the Four Corners region. One of his leading geologists, Clarence Dutton, recognized the Grand Staircase: a series of cliff-forming rock layers that step up northward from the Grand Canyon, each younger than the one below. The Claron Formation, which makes up most of the rock exposed at Cedar Breaks, is among the younger steps.

The rock layers

Mesozoic rocks lie beneath almost the whole monument, with younger Tertiary and Quaternary rocks capping the high points — all laid bare by intense erosion.

An amphitheater in retreat

The cliffs, spires and pinnacles were carved by erosion, weathering and mass wasting along the plateau's western edge. Much of today's topography comes from headward erosion of the Ashdown Creek drainage cutting back into the plateau rim. Several forces work together:

• Frost wedging: water freezing and thawing in cracks pries the rock apart.
• Chemical weathering: groundwater and surface water alter and dissolve minerals, carving small steps into the Claron Formation.
• Gravity and flash floods: in heavy storms, loosened rock falls as talus at the foot of cliffs or is washed downslope by sheetwash and flash floods.

Across the Colorado Plateau, rivers cut through nearly flat sedimentary layers. At Cedar Breaks a resistant cap — the Claron's white limestone member — sits over softer rock. As the weaker red limestone below is worn back beneath the cap, the cap eventually collapses and the cliff face moves back. The monument's cliffs retreat astonishingly fast, for four reasons identified by Lindquist (1980): weathering is rapid, protective plant cover is sparse, the soft rocks erode quickly, and the red limestone is undercut quickly beneath its white cap.

Erosional features and exposed rock layers at Cedar Breaks (NPS photo).

How hoodoos form

A hoodoo is a column, pinnacle or pillar of rock left standing where harder rock caps softer rock. The Claron Formation's alternating layers of hard and soft rock appear to be crucial.

• Joints and fractures speed erosion, and the walls and ridges at Cedar Breaks closely follow the main joint directions in the rock (Brox 1961).
• But joints aren't everything. In weak beds, and with walls retreating so fast, joints add little extra weakness. Hoodoos tend to form on the crests of ridges between gullies in the Claron's red limestone, near the top of the escarpment. A sharp change separates fast-weathering slopes from much slower-weathering bedrock, so once a hoodoo starts forming, the process reinforces itself.

As at nearby Bryce Canyon National Park, the pinnacles range from under 40 feet (12 m) to 200 feet (61 m) or more, among arches, spires and natural bridges. Lindquist named three kinds:

• Primary hoodoos jut out from the escarpment at right angles, like walls.
• Secondary hoodoos branch at various angles from primary hoodoos or the slopes below them.
• Ridge hoodoos stand on ridge crests hundreds of meters from the main escarpment.

Hoodoos can also cluster into complexes that radiate outward.

Fossils

Rock formations in and near the monument hold corals, snails, clams and oysters, fish, turtles, dinosaurs, birds and mammals, plants and petrified wood. All fossils in the national parks are protected under the Paleontological Resources Preservation Act of 2009.

Part of a larger region

Cedar Breaks belongs to the Colorado Plateaus physiographic province and shares its geologic history and some of its characteristic formations with a region far larger than the monument. The Geologic Resources Inventory has produced digital geologic maps and a report for the park, and a soil resources inventory has also been completed; both are available through the NPS DataStore.

Sources

• National Park Service: "NPS Geodiversity Atlas—Cedar Breaks National Monument, Utah," citing Lindquist (1980), Brox (1961) and Engineers International Inc. (1980).