The midday "sun dagger" solstice and equinox marker on Fajada Butte in Chaco Canyon, New Mexico, is formed by three sandstone slabs that collimate sunlight onto two spiral petroglyphs. The slabs appear to be the result of a natural rockfall and not a construct of the Chacoan Anasazi. Although neither the rockfall nor the petroglyphs can be dated accurately, it is likely that the petroglyphs were designed after the rockfall by people who observed the details of the light pattern for several annual cycles. Copyright ?? 1982 AAAS.
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Anasazi solar marker: The use of a natural rockfall