Chaco Canyon

And the Western Interior Seaway

Cropping out in various places,

Rocks are what erosion erases.

—Grook* by Dick Gibson circa 1976

The structures built by the Chaco Culture people in a canyon in what is now northwestern New Mexico were constructed beginning in the mid-800s (in Common Era year reckoning), and their culture flourished for about 300 years. The rocks that comprise the canyon and that the Chaco people used to construct their stone buildings were laid down during Cretaceous time, about 75 to 81 million years ago.

Chaco Canyon is cut into near-horizontal layers of the Mesa Verde Group of rocks, specifically the Menefee Formation and the Cliff House Sandstone. Those packages of rock represent deposits from rivers, mud flats, lakes, swamps, and beaches near the margin of the Cretaceous Western Interior Seaway as well as sediments deposited in the marine environment itself.

The Western Interior Seaway was a relatively shallow sea that extended continuously from the Arctic to the Gulf of Mexico along the front of the then-new Rocky Mountains. At its maximum, it reached from northeastern Arizona to Iowa and from western Alberta to western Ontario. Active erosion from the uplifted mountains carried huge amounts of sediment into the seaway, and that’s what is preserved in the rocks around Chaco Canyon.

The Menefee Formation is the more continental of the two packages of rocks, including river sands, overbank muds, and swampy coal beds. Those non-resistant rocks form slopes and low hills, and they contain abundant fossils indicative of the sub-tropical, wet, marine margin – leaves, crocodiles, turtles.

The Cliff House Sandstone is mostly sandstone from beaches and barrier islands just offshore in the Seaway. It forms thick resistant beds, and most of the 80- to 100-foot cliffs in Chaco Canyon are composed of it. Much of it was formed quite near the shore in shallow water, shown by ripple marks in the sandstone and burrows from clams and other organisms. But there are also shark teeth and ammonites, animals that lived in open water (though the teeth and shells could of course have been washed up on to beaches to be preserved in the sediment).

The erosion that scoured the region to make Chaco Canyon is recent, probably mostly from glacial times (the past two million years or so) when there were no glaciers here, but abundant rainfall would have caused significant erosion. Erosion continues to this day – Pueblo Bonito was built adjacent to a precariously poised slice of cliff that the Chacoans recognized as a threat (they built a retaining wall against it) and that the Park Service named “Threatening Rock.” Despite precautions, it collapsed on January 22, 1941, and destroyed part of Pueblo Bonito. Erosion always wins.

The layers of the Cretaceous rocks are pretty thin, and they weather into flat slabs, making them ideal for building construction by piling them up and chinking the spaces between with smaller flat pieces. That’s what the Chacoans did, quite artistically, as you can see in my photos from a trip there in 1987. The small chinking pieces probably came from the Menefee Formation while the larger slabs are Cliff House Sandstone.

The reasons for the decline of the Chaco culture and abandonment of the settlements in Chaco Canyon are complex and still debated, but there is little doubt that a major drought period beginning about 1130 (a.d.) and lasting for at least 50 years was an important factor.

*A grook is a short aphoristic poem invented by Danish polymath Piet Hein (1905-1996).