Chaco Canyon (Shallow Thoughts)

Akkana's Musings on Open Source Computing and Technology, Science, and Nature.

Wed, 20 Oct 2004

Chaco Canyon

I've been curious about Chaco Canyon ever since as a kid I read an article in Sky & Telescope about the Anasazi Sun Dagger, a rock structure whereby at the solstices and equinoxes the sun creates a narrow sliver of light projected onto a spiral petroglyph.

Unfortunately, it turns out that the Sun Dagger is not open to visitation (by the public or even by most researchers). In the 1980's it was deemed too fragile for visitors, and the site was closed down. There are some other astronomically oriented petroglyphs, but no one seems to know exactly where, or to have a complete list.

Getting information on Chaco is a bit difficult. There's not much useful information on the web, the park doesn't have specialized handouts like a lot of other parks (many parks have one-page handouts available for the asking on subjects such as geology, astronomy, petroglyphs, etc) except for one giving a brief listing of the available hiking trails. The ranger at the visitor's station was somewhat reticent: he recommended a couple of hiking trails, and told us that the Sun Dagger was located high on Fajada Butte, but not much more. I noticed a picture of some petroglyphs thought to depict a supernova, and asked where they were, but he apologized "Sorry, I don't go out that trail much".

Nothing to do but go try some stuff and see what's cool. We visited all the ruins along the park road, then headed up the steep trail to Pueblo Alto and the Pueblo Bonito overlook, which begins by a scramble through switchbacks over broken rocks, followed by a steep ascent through a narrow gap in the rock wall. Fun! And daunting: but it turns out that once you squeeze through the gap, you're up on top of the mesa and mostly finished with climbing.

The mesa top is interesting rock: white, layered mudstone, full of interesting embedded objects (presumably plant fossils, though some of them actually look like bone). The Fajada Butte interpretive sign, the only mention we found of park geology, says of the butte: Cliff House Sandstone forms the upper layer with deposits of fossil shells, clams, shark teeth, and marine sand. None of these fossils seem to correspond with what we saw embedded in the rock along the Pueblo Alto trail. More research is required.

The view of Pueblo Bonito from above is marvelous and well worth the short and interesting hike. The semicircular shape of the great house, not obvious from below, is striking when viewed from above.

The hike up to Pueblo Alto was pretty, and enjoyable as a hike, but Pueblo Alto itself is much less interesting than the ruins down in the canyon. We wished we'd gone the other way on the loop trail for more birds-eye views of the canyon houses.

Another interesting aspect of Chaco: their astronomy program. They have a fixed observatory (a dome housing a truss-tube dobsonian of about 18") and something outside on a tripod (probably a big Schmidt-Cassegrain). The visitor's center was full of photos of astronomical objects, as well as some information about light pollution. It's nice to see a park so interested in astronomy, especially with the sort of skies they must get at Chaco. Alas, we weren't able to stay the night.

But Chaco's big mystery is the "roads". The park literature talks about the amazing roads the Chacoans built, stretching for hundreds of miles between Chaco and neighboring settlements in many directions, used for trade between tribes. On the Pueblo Alto hike, a short segment of one such "road" is roped off and signed: a wide rectangle of more or less bare rock, perhaps ten or fifteen feet on a side, lined generously with rocks on two sides. With a lot of imagination, you could imagine a boulevard continuing in this fashion, rocks lining the left and right sides of the "road" like a huge version of some national park trails.

Dave smelled a rat, and dug further. These "roads", apparently, were originally detected as unexplained straight lines appearing in infra-red images, using NASA's TIMS system. Archaeologists subsequently searched the ground and found some short segments which looked vaguely road-like, and drew maps connecting the segments. Here's one such map of the Chaco road system. Notice anything unusual? Like the fact that the ground map doesn't actually match the lines in the IR image? Note also how straight the "roads" are in both theories.

It gets even weirder. One of the park's roadside pullouts points to a "Chacoan stairway" high on a mesa, and comments that the stairway was part of one of the roadways. The stairway is there, and it's neat. There are other stairways elsewhere in the park -- we saw photos (though the one section we saw up close, on the Pueblo Alto hike, was a bit too subtle for either of us to find the "stairway" on the indicated rock).

Why would the Chacoans build roads like this? It makes no sense. Why would a prehistoric people with no wagons or pack animals need rock-lined ten foot wide "roads", arrow straight and made without respect to the local topography?

Let's look at this practically. You're a Chacoan heading out to trade with someone in a pueblo to the south, or a southern resident travelling to Chaco. You have a choice between following a straight road, which requires you to climb up onto an 800 foot mesa, then down a precipitous set of rock stairs which lead to a steep scramble back down to the canyon bottom; or you can walk a quarter mile west and stroll through the huge gap between two mesas, without having to climb or descend at all. You're travelling on foot, carrying your pottery or baskets or whatever it is you're bringing to trade. Perhaps you have your family and kids along. Which route would you choose?

The stairways are there; and the "road" segments are there, too. But that doesn't mean that they connected to form hundred-mile long roads between communities. The stairways are useful for locals who want access to the mesa tops -- perhaps for defense, or religious purposes, or just for sightseeing. The short "road" segments on the ground -- who knows? Perhaps parade grounds. Or maybe they were malls, where vendors lined up to spread their wares out for customers to view. There are lots of possible explanations!


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[ 22:28 Oct 20, 2004    More travel/anasazi | permalink to this entry | ]

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