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As you explore the geology of the park, you are likely to discover evidence of ancient life far older than the monumental buildings that remind of us of the long history that Chaco Canyon preserves. The massive sandstone cliffs that make up the canyon were once part of a dynamic coastline that existed in northwest New Mexico between 85 and 75 million years ago. Situated at the edge of the expansive Western Interior Seaway, Chaco was once home to a diverse population of marine and land dwelling creatures, some of which have long been extinct. If you look closely at the rock around you, you may discover fossilized remains of the plants and animals that flourished here.
The Menefee Formation, made up mostly of shale and mudstone that underlies the large massive cliffs, represents an ancient coastal plain environment and contains fossil remains of terrestrial life like large coniferous trees, turtles, and fragmentary evidence of larger creatures like hadrosaur, theropod, and ceratopsian dinosaurs. Plant fossils include leaf impressions of palm and conifer, as well as specimens closely resembling modern laurel, witch-hazel, and camellia. The presence of these fossils suggests that the environment of the time was warm, moist, and subtropical.
The Cliff House Formation is the large cliff-forming unit in the park and represents an ancient shallow beach environment. Embedded in the sandstone throughout the canyon are casts and molds of clam shells, marine snails, ammonites, shark teeth, and bone fragments of large predatory marine lizards. The most common and often perplexing fossil at Chaco is a knobby, tubular, reddish-brown cast or mold that resembles rusty iron rebar or a fossilized corncob. Paleontologists call these knobby tubes Ophiomorpha nodosa (meaning “nodular dwelling place”) and believe they are the fossilized burrows of a shrimp-like crustacean known as Callianasa major.
These small crustaceans made their homes in the sand by burrowing into the near-shore seabed, creating a network of tunnels. In order to keep their tunnels from collapsing by constant wave action, the crustaceans cemented sand particles into the walls of the tunnels, making the tunnels knobby. In deeper waters, where sands were more stable, tunnels needed less cementing and were smooth.
When the ocean receded, heavy particles of iron, mercury, and other minerals suspended in the water moved downward and settled into openings that remained in the seabed. The iron particles provided structure, color, and texture to the crustaceans’ abandoned tunnels; these tunnels hardened as the seabed continued to dry.
As you continue your journey through Chaco, think about all the builders that have dwelled here, and enjoy and respect all of their creations. This truly is an extraordinary place. Please help us preserve and protect it by not moving or collecting fossils.
