The building are all stacked stone - astonishing masonry skills which in some cases are still standing and relatively intact after over 1000 years. A friend calls this place the "Machu Pichu" of the north. After being there I think he's right. It's a complex and extensive system of structures and roads which at it's peak they estimate supported 8000 people.
If you've never gone - you should. It's worth the hassle to get there. Here is a link to learn more: Chaco Culture. Also, here are a few pictures I took --