Salmon Ruins Sign
One of the most interesting chapters in Astronomy and Ceremony in the Prehistoric Southwest: Revisited reports on archaeoastronomical research at Salmon Pueblo (also known as Salmon Ruins), a large Chacoan “outlier” great house near modern Bloomfield, New Mexico. The paper is by Brooks Marshall and Larry Baker of the San Juan County Museum Association, which manages the site, and it argues that one particular room at Salmon was likely used as an observation station for both solar and lunar events.
The room, known as Room 82, is in the southeast corner of the central room block on the north side of the plaza, just southeast of the elevated “tower kiva” in the center of the block. When it was excavated in the 1970s, the excavators found that it had several unusual features that suggested it was used for specialized non-domestic purposes, though it was not clear to them at the time what those might be. In addition to the commonly found hearths, milling bins, and T-shaped doorways, there was an unusual opening in the east wall which Marshall and Baker call a “window” (unfortunately they don’t explain why they use this term, which is generally not used in describing Chacoan architecture), an adobe platform in the northwest corner that had two shallow pits at its north end, and a dividing wall in between of uncertain original height. The platform and wall were destroyed in the course of excavation, while the window has deteriorated a bit over time but is still there.
The unusual nature of these features and their east-west alignment made Marshall and Baker suspect an astronomical role, so in 2008 they created a replica of the adobe platform out of plywood and positioned it where they calculated it would be hit by light through the window at equivalent times to the original, taking into consideration the higher floor level due to backfilling of the room. In 2009, a stabilization project removed the backfill and allowed them to place the replica in the original location of the platform to verify their results. Further testing in 2010 and 2011 involved simulating the original size and shape of the window opening. Throughout these tests they placed two rocks on the platform to simulate the two pits at the north end of the original, known as Features 71 and 72.
Central Roomblock at Salmon Ruin
Their results were striking. They found that the northernmost pit, Feature 72, was lit by sunlight coming through the window only on the summer solstice, and when the original window opening was simulated the light was quite narrowly focused on this feature. Feature 71, despite being only a few centimeters away, was never lit up by the sun at all. It was, however, lit by moonlight during the major lunar standstill, when the moon rises at its most extreme position relative to the sun for a few years. It’s long been known that the first major period of construction at Salmon in AD 1089 and 1090 corresponds to the lead-up to a major lunar standstill, and indeed Marshall and Baker’s calculations showed that it was in these years that moonlight would have first illuminated the features at the north end of the platform. By the standstill itself, which lasted from AD 1093 to 1095, moonlight would have hit about three quarters of the platform. Marshall and Baker propose that the south end, which is never nit by moonlight, may have served as an observation point where someone could sit and observe the moonlight move across the platform over time, possibly allowing the prediction of the standstill.
Obviously the wall in between the window and the platform adds a complication to all this, as it would have blocked at least some light from coming through. There’s no way to tell how high it initially was, so Marshall and Baker ran some calculations based on different heights to see how they effected the illumination patterns they documented. They found that at a certain height the wall would have prevented the beam of light coming through the window from moving beyond the platform onto the floor, which may have been intentional. At higher heights it would have blocked the beam entirely, but the base of the wall was fairly thin and probably couldn’t have supported a full-height wall.
Excavated Rooms at Salmon Ruin
This is exciting research for several reasons. It’s always interesting to see a careful study of architectural alignments to celestial phenomena that takes into consideration excavation history and possible confounding factors. It’s particularly interesting that this study seems to have shown strong evidence for alignments even taking those other factors into consideration. The solstice alignment seems like the best established to me, which is unsurprising since solstice alignments in general are the best documented phenomena in ancient Pueblo archaeoastronomy. What I find most intriguing, however, is the possible lunar alignment and its relation to the construction dates at Salmon, since despite a lot of talk about lunar alignments at Chacoan sites very few have been securely documented, and unlike solar alignments there is no support for them in modern Pueblo ethnography. If this lunar alignment really does hold up, it helps strengthen the argument that the Chacoans really did observe and care about these subtle lunar cycles.
The strongest evidence so far for Chacoan observance of lunar standstill cycles comes from Chimney Rock Pueblo, further north in Colorado. The evidence here is quite strong indeed, as the full moon rises between twin spires of rock as seen from the great house only during the major lunar standstill, and this accounts for the otherwise very puzzling location of the great house atop a high, steep mesa. There is also some evidence from the dates of construction at Chimney Rock that some building periods were related to specific lunar cycles. The lack of support from other Chacoan sites, however, has made the seemingly solid evidence fr,om Chimney Rock hard to integrate into the picture as a whole. If people at Salmon, which is fairly close to Chimney Rock and is connected to it by an easy travel corridor along the San Juan River, were also marking the lunar standstill cycle, the picture begins to fill out a bit.
That said, this research is still fairly preliminary and I wouldn’t go as far as to say that the lunar alignment proposed here has been definitely established. More research is certainly necessary to confirm and interpret the patterns documented here. It is certainly suggestive, however, and very interesting.
Backfilled Rooms at Salmon Ruin
Gambler's House 