Shown is the "Sun Dagger" by which the Chacoans (Anasazi) were able to read the harvesting and planting seasons and recorded time's passage. At the winter solstice, rays of sunlight fell between the 2 huge stone slabs, neatly bracketing the spiral petroglyph on 443 foot Fajada Butte at the south entrance to Chaco Canyon. At the summer soltice, a single band of light bisects the center of the spiral. The spring and fall equinoxes were heralded by an additional light that fell on the smaller petroglyph, visible to the left of the larger one. This discover was made by Anna Sofaer in 1977.