Sunday, March 23, 2008

Sunday stroll: UFOs and alcoves

Just across the New Mexico border, near the town of Aztec, New Mexico, it is rumored that a UFO crashed on a sandstone cliff in 1948. The Aztec town library will be holding a UFO symposium next weekend, as it does every year. We headed out to the crash site - or at least to a trail near it - this weekend, so we could explore without being abducted.

We didn't see anything resembling alien spacecraft (though it's possible that the mountain bikers were in disguise). But if the aliens wanted to hide, we found some nice sapping alcoves for them.

If you look up at the cliffs of the canyon country of the American Southwest, you might see near-caves, overhanging sandstone that shades little alcoves. The Ancestral Puebloans (aka the Anasazi) made use of them to build cliff dwellings in places like Mesa Verde. There aren't any cliff dwellings in the sandstone above Aztec (although there are some other Ancestral Puebloan ruins nearby), but there are alcoves.

Alcoves are excavated by groundwater.* Water seeps into the sandstone at the surface, and then, somewhere below, it hits a less permeable layer and flows to the nearest open surface - in my part of the world, that would probably be a canyon wall. As the groundwater leaks out, it loosens the sands grains, and gradually removes rock beneath an overhang. (If the process continues, it can create arches like those in the parks to our west in Utah.)

I've seen more spectacular alcoves than those along the Aztec UFO trail - Mesa Verde, of course, but also in the Canyons of the Ancients National Monument. But I've never seen water leaking out of them before.

I don't know what caused the permeability of the rock to change. There was sandstone above, and sandstone below. But there was water trickling out of the rock - it has been a wet winter, after all - and it was carrying sand with it.

And I forgot to get out the camera, because my son thought the cave was a little scary.

We probably shouldn't have mentioned the UFO story to him.

Even though I didn't bring my camera, there are pictures of many of alcoves with ruins in them around. Here's one:

Sand Canyon, west of Cortez, Colorado. Source (with lots of other nice pictures and descriptions).

Different sandstone unit. Same hydrologic process.

* Edit: Or maybe not. See the comments for references to other explanations.


ScienceWoman said...

There are those that dispute that sapping is an effective process in bedrock. They argue that in every alcove (or at least every amphitheatre headed valley) they observe, that there was evidence of overland flow and plunge pool erosion. See Lamb et al. 2006. "Can springs cut canyons into rock?" JGR. E07002, doi:10.1029/2005JE002663

Kim said...

Oh, interesting! Thanks for pointing that out.

Now I really wish I had taken a picture. This particular alcove was tiny - my son wouldn't be able to crawl into it - and was no more than three feet below the top of the (tiny) cliff. The catchment area for any overland flow would have been pretty small (especially compared to places like Mesa Verde).

What would you look for? Is there a minimum amount of water necessary to drive the erosion?

coconino said...

I would argue that there is a combination of both overland flow and sapping for most alcoves. Monsoon events in the desert southwest, even for small catchments, will produce considerable overland flow (occasional inch/hour storms can be frequent as was the norm during monsoon 2006). I seem to recollect that the overland flow during brief intense storms is of the Hortonian type, where the surface essentially seals and very limited infiltration occurs. Also could there be a synergistic effect between normal weathering and gw surfacing to enhance the solution of the gypsum or calcite cement?

coconino said...

You piqued my interest; I had to look around for carbonic acid's part in enhancing erosion in calcite-cemented sandstones. When gw surfaces, it combines with CO2 to form carbonic acid which, in turn, enhances the erosion process by dissolving the calcite cement. I hate to think of how the Ancient Puebloans teeth looked after 35 years of alcove/metate sand.