Hovenweep
Hovenweep is a site near Monticello, where southwestern Colorado meets southeastern Utah. It is an American National Monument, and majority of the structures found there were constructured in the early to mid 13th century AD by ancestral Puebloans(formerly Anasazi) part of the Montezuma Valley/Mesa Verde culture. Civilisation flourished in the Mesa Verde area from 900 AD to 1300 AD. The people of Hovenweep had well developed architectural, craft, farming and masonry skills, building many pueblos and cliff structures.
By the end of the 13th century the people of Hovenweep and the surrounding region (such as Mesa Verde and Kayenta) deserted the area and are presumed to have joined the people of the Hopi and Zuni in the south. Suggested reasons for their departure include hostile neighbors, overpopulation, overuse of the land, and a long drought which began in 1276 AD.
Holly House ruin is an outlying site at Hovenweep. About 100 meters south of it in the canyon below are 2 large boulders, one with an overhanging ledge. On the smooth lower face of the overhang are a row of 3 petroglyphs - a set of concentric circles believed to be a sun symbol, and 2 spirals on the set's left.
45 minutes after sunrise on the summer solstice, a ray of light appears at the side of the leftmost spiral and elongates to the right, cutting the top part of the spiral. Light appears to the right of the middle spiral, between it and the sun symbol, stretching horizontally in both directions and almost bisects the right spiral. A 3rd ray of light appears at the right of the sun symbol, moves left and cleanly bisects it. Finally, all the light rays merge and a light shaft stretches the whole length of the boulder. All this takes place in 10 minutes.
This observation was discovered by Dr Ray Williamson. Such intricate and accurate petroglyph positioning can probably be attributed to observant shamans who ceaselessly noted their surroundings and the recurrent patterns around them.