Rising 135 meters above the canyon floor, Fajada butte contains
the well-known three slabs and spiral petroglyphs which mark the cycles of both sun and moon. The 2-3 meter sandstone slabs cast shadows of the late morning and midday sun to indicate both solstices and equinoxes.
When these light and shadow phenomena were first discovered, a vertical shaft of light passed through the center of the spiral at summer solstice. At winter solstice, two noonday daggers framed the large spiral. During the equinoxes, the smaller spiral was bisected at midday by a lesser dagger.
A moonshadow bisects the spiral at moonrise during minor northern standstill and just touches the petroglyph's left edge during major northern standstill. At both places on the petroglyph a straight groove has been pecked which is parallel to the moon's shadow. Since the groove is parallel to the moon's shadow, it does seem likely that Chacoan astronomers were aware of the major and minor standstills of the moon. The three-slab site is unique in the ancient world in marking on one surface the significant cycles of both the sun and moon.
Within recent years a settling of the slabs has occurred, probably now halted. The beam of light no longer crosses the exact center of the spiral at summer solstice. One of the double daggers that was initially tangent to the large spiral at winter solstice has disappeared due to a shift by several centimeters of the middle and eastern slabs.
A 230 m long ramp, which may have ascended the 95 m to the top of the butte, appears associated with a Post-Chaco (A. D. 1150-1250)
structure on its upper ledges.
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