Stand on the rim of Bryce Canyon National Park in southern Utah and look out across the amphitheatre. Below you, rising hundreds of metres from the canyon floor, are tens of thousands of slender rock spires — colourful, irregularly shaped, sometimes thin enough to seem ready to topple in the next wind. They are arranged in dense thickets, like rocky forests, with corridors and side canyons winding among them. The colours range from deep orange to pink to brown to white, often with horizontal bands marking different rock layers. These spires are hoodoos — one of the more distinctive and recognisable landform types in physical geography. Hoodoos are formed by a particular kind of differential erosion, where a hard cap rock protects softer material below from weathering. Where the cap rock has been undermined by undercutting, the spire below collapses. Where the cap rock is still in place, the spire continues to rise as the surrounding less-protected rock is eroded away. The slow process of cap rock protection and eventual collapse has produced one of the most photographable and visited landscapes on Earth.
The Word and the Original Meaning
"Hoodoo" has a complex etymology. It may derive from "voodoo" (the West African spiritual tradition brought to the Americas through the slave trade) — both words share a sense of supernatural strangeness, and the strange-looking hoodoo rocks were sometimes attributed to supernatural forces. The original American usage of "hoodoo" as a name for these rocks dates to at least the 1860s. The Paiute name for the Bryce Canyon hoodoos was "Anka-ku-was-a-wits," meaning "red painted faces" — a name that referenced the strikingly painted faces of the hoodoos when seen from a distance and partly resembling figures. Indigenous mythology in various Western American tribes attributed the hoodoos to ancestor spirits, transformed people, or various other supernatural origins. The geological explanation came later, with the understanding of differential erosion processes.
How Hoodoos Form
Hoodoo formation requires a specific geological setup: a hard, weather-resistant cap rock overlying softer, more weatherable rock below. The hard cap rock can be limestone, dolomite, sandstone, or various other materials. The softer rock below can be shale, mudstone, weak sandstone, or limestone with different weathering properties. Water from rainfall and snowmelt erodes the softer rock from beneath the cap, undercutting it. Eventually, the cap rock may collapse if the undercutting becomes too severe. But while the cap is still in place, it protects a column of softer rock beneath it from being eroded away. The surrounding terrain — without cap rock protection — is being eroded away at a faster rate. Over millennia, the protected column slowly rises as a thin spire above the surrounding eroded terrain. The hoodoo is essentially a fossil of the original ground level, protected by its cap of resistant rock.
Bryce Canyon
Bryce Canyon is the type location for hoodoos. The park covers approximately 145 square kilometres on the eastern edge of the Paunsaugunt Plateau in southern Utah. The hoodoos form in the Claron Formation — a sequence of limestone, mudstone, and sandstone deposited approximately 50 million years ago in shallow inland lakes and marshes. The various rock layers within the Claron Formation have different weathering properties, with the more resistant limestone layers serving as cap rocks for hoodoos. The colours of the rocks reflect iron content — high iron content produces the deep orange and red colours, low iron content produces the white and pink hues. The horizontal banding visible on the hoodoos reflects the original sedimentary layering of the Claron Formation. Bryce Canyon was carved by erosion that proceeded along joints in the rock; the major fingers of erosion produced the amphitheatres, and the surviving columns of more-resistant rock became the hoodoos.
The Bryce Hoodoo Population
Bryce Canyon has perhaps a million distinct hoodoos. The largest ones — like the "Thor's Hammer" formation that has become a symbol of the park — are tens of metres tall. The smallest are perhaps a metre tall. The hoodoos are arranged in dense thickets covering the amphitheatre floors. The amphitheatres themselves have evocative names: Bryce Amphitheater (the largest), Black Birch Canyon, Bristlecone Loop, Fairyland Loop, Inspiration Point, Sunset Point, Sunrise Point. Each amphitheatre has its own characteristic shape and hoodoo distribution. Standard hiking trails wind through the hoodoo forests, allowing visitors to walk among the formations and appreciate their scale. The Navajo Loop and Wall Street trails are particularly popular for the experience of walking among the hoodoos.
Other Hoodoo Locations
Hoodoos occur in many other locations beyond Bryce Canyon, though usually less densely concentrated. The Goblin Valley State Park in southern Utah has thousands of hoodoo-like formations called "goblins" — somewhat shorter and rounder than the Bryce hoodoos but produced by similar processes. Cappadocia in Turkey has hoodoo-type formations (called "fairy chimneys") in volcanic tuff — soft tuff is eroded away while harder basalt cap rocks protect the underlying columns. The Cappadocian fairy chimneys have been carved out by humans for over a millennium into houses, churches, and entire underground cities. The Hoodoos of Alberta in southern Alberta, Canada have substantial hoodoo formations in sandstone and mudstone. The "Hoodoos" trail near Banff, Alberta features distinctive examples. The "Stone Forest" (Shilin) of Yunnan Province, China features hoodoo-like limestone columns produced by karst processes. The Coumbo Bluffs of Lassen Volcanic National Park, the Demoiselles coiffées (coiffed maidens) of the French Alps, and various other locations around the world have hoodoo-type formations.
Cappadocia's Fairy Chimneys
The "fairy chimneys" of Cappadocia in central Turkey are perhaps the most culturally significant hoodoo-type formations in the world. They were formed by volcanic tuff (the soft, easily eroded ash deposit from ancient eruptions) being eroded around more-resistant basalt cap rocks. The result is thousands of conical columns, often with the basalt cap clearly visible at the top. Over the past 2,000+ years, Anatolian inhabitants have carved homes, churches, and entire underground cities into the soft tuff. Some of the most impressive Cappadocian sites — the underground cities of Derinkuyu and Kaymaklı, the cave churches of Göreme, the rock-hewn dwellings of various villages — were carved into hoodoo-type rocks. The combination of natural rock formations and human modification has produced one of the most distinctive cultural landscapes in the world. The fairy chimneys have been a UNESCO World Heritage Site since 1985.
Differential Erosion and Cap Rocks
Hoodoo formation is a particularly visible example of differential erosion — the principle that softer rock erodes faster than harder rock. The same principle produces many other landforms: mesa landscapes (where a hard cap protects a broad area below), pedestal rocks (where smaller cap rocks protect short columns), arches (where erosion has broken through a thin layer of rock leaving an arched fragment behind), natural bridges, and various other features. The specific geometry of a hoodoo — tall, thin, with a distinct cap — reflects a particular ratio of cap-rock resistance to underlying-rock erodibility, and a particular history of undercutting and erosion. In landscapes where the cap rock is more resistant relative to the underlying rock, the hoodoos can be very thin and tall. In landscapes where the cap rock is less resistant, the hoodoos are stubbier or more like pedestal rocks.
The Bryce Climate Connection
Bryce Canyon's hoodoos depend on a specific combination of climate and rock conditions. The high elevation of the Paunsaugunt Plateau (around 2,400 metres) produces cold winters with substantial snow. The freeze-thaw cycling of winter water expanding and contracting in cracks is one of the major mechanisms producing the hoodoos. Liquid water enters cracks during warmer periods; freezes and expands during cold periods; opens the cracks further; melts and drains away during warming. Over thousands of years, the freeze-thaw cycling cracks apart the underlying rock and undermines the hoodoos. The cap rocks fall off, the columns collapse, new hoodoos form from the unaffected rock around them. The process is continuous — Bryce Canyon's hoodoos are not static. They are constantly being modified by ongoing erosion, with old hoodoos collapsing and new ones being exposed by the gradual erosion of the surrounding terrain.
The Pace of Change
The exact rate of hoodoo formation and destruction at Bryce Canyon has been estimated as roughly 2-4 millimetres of erosion per year. At this rate, the average hoodoo lifetime is on the order of 1-10 millennia, depending on its size and the strength of its cap rock. Individual hoodoos can be observed changing over even decadal timescales — sections fall off, new cracks appear, sometimes entire columns collapse. The famous Thor's Hammer formation has a clearly fragile balance between its cap rock and its support column; the formation has been monitored and is showing signs of progressive weakening. The eventual collapse of any individual hoodoo will be replaced by the formation of new ones from the gradually-eroding plateau material. The hoodoo "forest" at Bryce Canyon is therefore a dynamic feature, with the population of individual hoodoos turning over on timescales of thousands of years even as the overall landscape remains hoodoo-rich.
What They Mean
Hoodoos are striking examples of how geology produces the unexpected. The combination of differential erosion, freeze-thaw weathering, and the specific rock layering of the Claron Formation produces a landscape that looks unlike anything else on Earth. The result is a UNESCO World Heritage Site that attracts millions of visitors per year. Beyond their tourism value, hoodoos are scientifically interesting because they preserve information about past landscapes — each hoodoo represents a fossil of the ground level when its cap rock first became a protector. The vertical distribution of hoodoos at Bryce Canyon records the gradual erosion of the plateau over the past several million years. They are also reminders of how geological processes that seem slow on human timescales can produce dramatic landscapes over geological timescales. The hoodoos of Bryce, Cappadocia, and other locations are the visible expression of forces that have been acting on the Earth's surface continuously for millions of years.