"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" ...from this website http://www.colorado.edu/Conferences/chaco/tour/fajada.htm