In 1977 the artist Anna Sofaer visited Chaco Canyon as a volunteer recording rock art. There she recorded petroglyphs on Fajada Butte at what is now called the Sun Dagger site, now perhaps the most famous site in Chaco Canyon, located at a southeastern facing cliff near the top of Fajada Butte. She noted three large stone slabs leaning against the cliff which channel light and shadow markings onto two spiral petroglyphs on the cliff wall. On her second visit she saw a "dagger of light" bisecting one of the spirals. At about 11:15 am. on the summer solstice a dagger-shaped light form pierces the larger of the two spirals (Sofaer, Zinser and Sinclair 1979, p. 285). Similar sun daggers mark the winter solstice and equinoxes {Sofaer, Zinser and Sinclair 1979, p. 286} At one extreme in the moon's eighteen- to nineteen-year cycle (the lunar minor standstill), a shadow bisects the larger spiral just as the moon rises; and at its other extreme, nine-and-a-half years later (the lunar major standstill), the shadow of the rising moon falls on the left edge of the larger spiral.(Sofaer, Sinclair and Doggett 1982, p. 43) In each case these shadows align with pecked grooves (Sofaer and Sinclair 1987, pp. 48 – 59). Due to one of the slabs settling, the "dagger of light" no longer crosses through the center of the spiral during the summer solstice. Public access to the butte was curtailed when, in 1989, erosion from modern foot traffic was found to be responsible for one of the three screening slabs at the "Sun Dagger" site shifting out of its ancient position. Because of that shift, the assemblage of stones has lost some of its former spatial and temporal precision as a solar and lunar calendar. In 1990 the screens were stabilized and placed under observation, but the wayward slab was not moved back into its original orientation.