Hovenweep

Stone towers standing on the rims and boulders of shallow canyons on the Utah and Colorado line, built by Ancestral Puebloans mostly between 1230 and 1275 when about 2,500 people lived here. The masonry is precisely fitted and set with a mortar of sand, ash, clay and water. The towers are the interesting part: the slots and doorways of Hovenweep Castle in the Square Tower Group have been shown to work as a solar calendar, with light coming through the openings at sunset to land on interior door lintels at the summer solstice, the winter solstice and both equinoxes. They left in the late thirteenth century, most likely driven out by a 23-year drought, and went south to the Hopi mesas and the Rio Grande pueblos. Designated a national monument on 2 March 1923 and an International Dark Sky Park in July 2014, which is the other reason to come.