Chaco's main draw is Pueblo Bonito, one of the most extensively excavated and studied sites in North America. Center of the Chacoan world and occupied from the mid-800s to 1200s, it was a four-story masonry "great house" with more than 600 rooms and 40 kivas.
As we learned from our college-student ranger-in-training, at the height of its culture, about 1050 A.D., Chaco was probably the ceremonial, administrative and economic center for far-flung communities connected to it by 1,200 miles of roads.
We climbed through the now-deserted rooms, stooping to enter doorways, as he pointed out original wood-beam ceilings and explained how structures were often oriented to solar or lunar events and to cardinal directions. The only restoration has been shoring up a few parts damaged by a rockfall.
The tour was free (Pueblo Bonito is the only site at Chaco where such guided tours are available) and you can also wander on your own, picking up written guides at each of the sites.
Mesa Verde's major ruins are in alcoves set into cliffs under natural overhangs, and that means more huffing and puffing to see them: Many require strenuous climbs on canyon trails and up ladders, through tunnels and down into below-ground kivas.
Mesa Verde's classic period was between 1100 and 1300, later than Chaco's. Cliff Palace is Mesa Verde's largest and best-known site, and North America's largest cliff dwelling, with 150 rooms (you climb five 8-10-foot ladders and can descend into one of its kivas). It was partially restored in an earlier era when people thought it would be nice to fix them up so visitors would get a sense of what they were once like.
Unlike at Chaco, we never had a moment alone at Mesa Verde. To protect the sites and regulate crowds, you must visit most of the sites with a ranger on a timed tour, for $3 per site.
...Read more about Chaco and Mesa Verde cliff dwellings and great houses.