We think of mountains as the very definition of solid and unchanging. They're the backdrop that stays put while everything else moves. So the idea of a 'walking mountain' sounds like something from a fairy tale. But in one corner of Colorado, the ground really is on the move.
Here's the truth. No mountain actually walks. But in the San Juan Mountains of southwestern Colorado, a giant slow-moving landslide called the Slumgullion Earthflow has been creeping down a valley for centuries. One active part of it has been moving continuously for about 300 years, at a rate of roughly 23 feet per year. It's a mountainside in very slow motion.
A Landslide That Never Stopped
The story starts about 700 years ago. A huge mass of earth crawled down the valley near what is now Lake City, Colorado, creating a tongue of debris about four miles long and 2,000 feet wide.
That first flow eventually came to rest. But a second earthflow began about 300 years ago, and it's still going, sliding over older, stable rock. Twenty-three feet a year doesn't sound like much, but over three centuries it adds up to a lot of moving mountainside.
The site is so remarkable that it was named a National Natural Landmark in 1983. It's described as a striking example of mass wasting, a geologist's term for the movement of large masses of earth material downhill. Colorado Highway 149 even crosses it.
How Earth Can Flow
An earthflow is exactly what it sounds like: soil and rock that flow downhill like an extremely thick liquid. It happens when fine-grained material, such as clay, silt, or fine sand, becomes soaked with water. The water reduces friction and weakens the ground, and gravity does the rest.
The speed depends mostly on how much water is in the ground. Some earthflows move fast enough to see in minutes. Others, like the Slumgullion, move so slowly that it takes years to notice. Wet periods, heavy rain, and melting snow make them more likely.
A Lake Made by a Landslide
The Slumgullion left something beautiful behind. When the first earthflow came down about 700 years ago, it blocked a river called the Lake Fork of the Gunnison. Water backed up behind the natural dam and formed Lake San Cristobal, about two miles long and up to 89 feet deep.
Natural landslide dams can fail, sometimes after a very long time, so the U.S. Geological Survey checked this one. It concluded the dam is stable. Back in the 1950s, though, the agency decided against building a larger artificial dam there, partly out of concern that the still-moving part of the earthflow might one day reach it.
Rocks That Walk on Their Own
If you want rocks that truly seem to wander, head to Death Valley National Park in California. On a dry lakebed called Racetrack Playa, stones leave long trails across the flat ground. They've been nicknamed sailing stones, or walking rocks.
For decades, nobody knew how they moved. Then, in 2014, researchers published time-lapse footage and GPS data. Thin sheets of ice on a shallow winter pond were breaking up on sunny mornings and, pushed by light winds, shoving the rocks along. Some moved more than 200 yards in one winter.
So while no mountain can pack up and leave, the Earth is never quite as still as it looks. Sometimes, if you're patient enough, you can almost watch the ground take a step.
