The "Sun Dagger" is a spiral petroglyph by which the Chacoans (previously known as "Anasazi") were able to read the seasons using a shaft of sunlight falling between two stone slabs; the spiral center marked the position of the sun at the summer solstice.  For photo and details click here:

http://www.lapahie.com/Chaco_Sun_Dagger.cfm
(Harrison Lapahie Jr.)