Photo: © Fritz Swanson, Creative Commons Attribution License (Wikipedia)
The 135 m tall Fajada Butte in the Chaco Canyon. It contains three 2 - 3 m high slabs and a spiral rock carving. The slabs are positioned in such a way that they cast their shadows over the spiral to indicate solstices and equinoxes.
The photo below shows the carving and the light from the "dagger" produced by the slabs at the time of the summer solstice.
Photo: © University of Colorado; reproduced by permission