We ditto what Roger said.
Farmerman, below is a link to an interesting paper on the markings of Fajada Butte for comments on the spiral grooves indicating the lunar cycle, including the entire cycle of 18.6 years.
Watching the night sky at Chaco, it's easy to understand why they studied the moon so extensively. In the desert, the moon is so bright that it lights up the earth as well as the sky. The shadows it casts are black and clearly delineated.
For me, it is pure magic.
http://www.solsticeproject.org/lunarmark.htm
We conclude that the Pueblo Indians recorded the extreme northern rising positions of the moon at major and minor standstills. In addition, as we speculated earlier (Sofaer et al. 1979 a), the number of grooves in the spiral (counting horizontally from the left edge to the right edge) may record the length of the cycle. This appears in two possible ways: (1) as the cycle moves from minor to major standstill over 9 to 10 years, the extreme position of the lunar shadow shifts over the 10 grooves on the left side of the spiral; (2) the length of the full cycle (18.6 years) may be recorded by the count of 19 grooves across the full spiral. The number of grooves may also record a knowledge of the 19-year Metonic cycle. In addition the passage of the shadow edge through the far right groove of the spiral may record the midpoint of the declination cycles of the sun and moon.