The intuitive explanation for an eroded landscape is always water. A river cuts, rain washes, the sea pounds. At Bryce Canyon that explanation does not work, and a glance at the map is enough to make you suspect it: there is no significant river crossing the park.
What carves these columns is a slower, more precise tool. Water gets into the cracks in the rock, freezes, increases in volume and acts as a wedge. The next day it thaws, seeps a little further in and freezes again. Here that cycle repeats some 170 days a year.
01 An ice wedge every other day
The process has a technical name: frost wedging. It is an elementary mechanism of materials physics. Water is one of the few substances that expand on freezing, by around 9 %, and that expansion, inside a closed fissure, exerts an enormous pressure.
What is exceptional about Bryce is not the mechanism, which happens on any mountain, but the frequency. Some 170 cycles a year means that on almost half the days of the year the rock freezes and thaws at least once. No rock withstands that indefinitely.
The reason for that frequency is altitude combined with latitude. With the park rim between 2,400 and 2,700 metres, the nights drop below freezing for much of the year, and the plateau sun warms the rock above freezing during the day. At the Bryce Canyon City station, at 2,336 metres, January's average low is −12.3 degrees and the same month's high is 2.1.
02 And it is not a canyon either
The second fact worth correcting is in the name. Bryce Canyon is not a canyon. A canyon is a valley dug by a central watercourse, with a river at the bottom. There is no such thing here.
What there is is a collection of giant amphitheatres, formed by headward erosion: the plateau rim eats backwards, and what is left in front are the columns that have not yet come apart. The largest of those amphitheatres, Bryce, is about 19 kilometres long, 5 wide and 240 metres deep, and the full series extends some 32 kilometres north to south.
The difference is not terminological. A canyon deepens; an amphitheatre retreats. Bryce is not getting deeper: it is moving backwards, and the columns you see today are the front of that advance.
03 The rock that lets itself be carved
For ice to produce columns rather than simply rubble, you need the right rock. Bryce's is called the Claron formation, and it is the one that gives the pink cliffs, the highest and youngest step of southern Utah's Grand Staircase of layers.
Its characteristic is that it is not homogeneous: it alternates harder layers with softer ones. When ice opens a vertical fissure, erosion takes the soft material first and leaves the hard standing out. Hence the stepped shapes and, above all, the blocks that sit on top of some columns like a hat.
That capping block is not decorative: it is the reason the column exists. As long as the hard layer on top holds, it shelters what is below it from the rain. When the block falls, the column comes apart in a few decades.
04 Between sixty and a hundred and twenty centimetres a century
The speed of the process has been measured. Under natural conditions, Bryce's hoodoos retreat between two and four feet a century, that is, between 60 and 120 centimetres every hundred years.
Put in perspective, that is extremely fast for stone. It means that a column 60 metres tall, the maximum the largest reach, cannot have been there much longer than a few thousand years. The landscape photographed at Bryce is geologically recent and provisional.
It also means that none of the columns standing today will be there in a few centuries, and that the ones that will be visible then are still inside the wall, waiting for the ice to separate them.
05 The freeze-thaw days are falling
There is one monitoring figure worth knowing. The Park Service has measured freeze-thaw days at Bryce since the 1960s, and the series show a decline: from some 220 days a year in those first measurements to some 170 in 2020.
The logical consequence is that the tool that manufactures the hoodoos is working fewer hours. What effect that eventually has on the shapes is not known with certainty, and the Park Service itself frames it as an open question rather than a forecast.
What is certain is that these columns depend on a very narrow condition: nights below freezing and days above, repeated hundreds of times. It is a landscape that exists thanks to a temperature threshold, not thanks to durable rock.
06 How to look at it
The first practical consequence is that it pays to go down. From the rim you see the shape of the amphitheatre, but the mechanism is understood at the bottom, among the columns, where you can see the vertical fissures, the capping blocks and the recent rubble at the foot of each wall.
The second is that winter is not Bryce's bad season: it is the season in which the landscape is working. In January and February 44.7 centimetres of snow fall each month, and that snow is exactly the raw material of the process. Seeing it under snow is not a meteorological accident: it is seeing it in operation.
And the third is the one that changes how you look. A canyon is a story about a river. Bryce is a story about thermometers: about 170 nights a year in which water trapped in a crack freezes and pushes a millimetre further. Everything photographed there is the accumulated result of that millimetre.
