The dunes look out of place.
The Sangre de Christo Mountains are postcard Colorado. They stand in formation with a clearly defined alpine tree line and a higher snow line. They corner the dunes from the north and east. Flat grassland and bushes stretch south and west.
The video in the park visitor center explained that the sand had been blown up against the mountain range from the prairie. Snowmelt and rain water washed the sand down from the mountainside, and the wind swept it back up there. These competing forces funneled the sand into a pocket created by the mountains, thus creating the tallest sand dunes in North America – 750 feet above the surrounding area. Despite their well reasoned and clearly presented explanation, even the park service concedes the existence of the dunes is funky.
There weren’t any campsites available within the park but luckily they allow backcountry camping in the interior of the dunes. After getting set up with a permit, I drove down to Medano Creek to kill some time before it cooled off enough to hike in.
Medano Creek exhibits a rare phenomena known as surge flow, which are basically waves of increased flowrate moving down stream.
There is a video on the park website which explains the process. Dams of sand (probably only a few inches tall) are created naturally by the creek. Successive dams create pools which flow over and into one another. When an upstream dam reaches its breakthrough point it rapidly disintegrates releasing its volume down into the pool below it. The lower pool quickly exceeds its breakthrough point and its volume is added to the pulse of increased flow. I didn’t catch how far upstream these waves are thought to originate or what exactly causes the first dam to reaches its breakthrough point. This youtube video shows the surge flow.
Pulse flow is a subtle quirk, but the more I thought about it, the more curious I became about the required conditions and mechanism at work. It was as peacefully hypnotic as it was rigorously interesting.
While packing up my backpack in preparation for the night, I made conversation with a pair of guys in the parking lot. It was their first time backcountry camping at the park too. I left just before they did, and I had about a 200 yard head start. After walking barefoot through Medano Creek, I began picking my way up the dunes. Years of experience dirtbiking in sand helped me navigate. It’s simple once you understand how to read the dunes and judge what looks like loose sand (leeward side) and what looks firm (windward side).
The two guys behind me fell off my pace, and I ended up about a mile away from them by the time we decided to make camp.
While walking on steep dunes during my hike in, the sand would groan underneath my boot.
It’s hard to explain what it sounded like except to say that it was as much a vibration through my foot as it was a vibration in my ear. “brrrrupt” or “fvvvvt” or “nmmmm” – short sounds that lasted only as long as my boot was slipping and sinking back downslope. It was peculiar, but after the first four or five times I got used to it and was able to contain my laughter.
After I had camp set up, I was trying to capture the best line-of-sight including my flag, the dunes, and the sunset, and I happened to back-peddle onto the crest of the dune I was on. A low hum began, and even after I had caught myself it continued.
Deeper and louder than the groans from my steps, it was coming from the length of the ridge. I looked down the leeward side, and because of the low light it took me a minute to resolve what I saw. I had started a landslide that was hardly perceptible with my eyes, but the vibrations underneath me were terrifying. It felt and sounded like the slow and grinding shudder of a freight train just before it comes to a complete stop, that last quarter second of motion before the air brakes relax. Except it was much longer. It’s hard to estimate time when something like this happens, but I would conservatively claim that the whole event went on for at least four seconds. I was irrationally and legitimately afraid that the dune was about to fluidize allowing me to fall straight down into it.
I did a little research, and it’s believed that “singing dunes” are the result of tiny grains of sand bouncing off of each other during a sand slide, cumulatively producing an audible noise. I think theres a bit more to it. I think that as the sand grains move around and reposition themselves they become more tightly packed and better consolidated. This could happen at deeper levels than the thin (maybe 1″ tall) avalanche on the surface. The grains of sand wiggle closer together within the dune, and the air they displace is forced upwards through a “tortuous path” (For example: The air flowing through a french horn follows a tortuous path). The tortuous path causes turbulence and vibrations. The upward moving air further jostles sand grains and produces better and better consolidation. In my mind, this would explain why the process was moderately self sustaining, instead of the short groans I had heard from my steps earlier. I felt a scared, surprised, and confounded type of excitement.
This is the best video I found on the phenomena.
The cloud cover never fully dispersed, and full moon was two or three days before, so the star gazing wasn’t great. After the post-sunset glow faded, my flag changed directions, and cool air washed in from the mountains. The two dudes from the parking lot must have seen me sorting around my camp with my headlamp. They signaled me by aiming their light in my direction and flashing it on and off a few times. I signaled back in kind.
I could go on about how I was blown off of the ridge around 2am, but I’m a little exhausted with writing this post. I’ll just say it was a wild 15 minutes wrangling everything and retreating a few hundred feet down the hill. You’ll just have to take my word for it.
The next morning I retraced my dull footprints out of the dunes and left the park before 9:30am.