From the World Heritage inscription:
A great concentration of ancestral Pueblo Indian dwellings, built from the 6th to the 12th century, can be found on the Mesa Verde plateau in south-west Colorado at an altitude of more than 2,600 m. Some 4,400 sites have been recorded, including villages built on the Mesa top. There are also imposing cliff dwellings, built of stone and comprising more than 100 rooms.
Mesa Verde is another really under appreciated national park. While is is one of the first national parks in the US, it is a cultural attraction, not a natural one. In fact, North America is seriously lacking in ancient structures (due do the nomadic nature of the original inhabitants) and Mesa Verde is one of the finest.