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The ‘sun dagger,’ a midday solstice and equinox marker on Fajada Butte in Chaco Canyon, New Mexico, evidently is not a construct of the Chacoan Anasazi, as originally thought. E.B. Newman and R.K. March of the United States Geological Survey and A.G. Vivian of the Arizona State Museum, Tucson, recently reported the results of their study of the sun dagger. They say that erosional processes, not Anasazi engineering, produced the solar marker (Science, Sept. 10, 1982).

It was difficult to understand how the Indian culture had been able to design and construct such a huge, and yet sophisticated, instrument in the 10th or 11th centuries, when other buildings were made of stones 2 orders of magnitude lighter. The Anasazi had indeed used the other stations as observatories to record sunrise and sunset, but the sun dagger was the first to be discovered that recorded midday solar observations. The geological evidence suggests that the large sandstone slabs that collimate sunlight and produce the sun dagger image are the result of a natural rockfall and that marker petroglyphs were designed at a later time to observe details of the light pattern for several annual cycles.