Aztec Ruins National Monument

Aztec Ruins National Monument was established in 1923 to preserve the remarkable remains of an ancestral Puebloan farming community, including a twelfth-century Chacoan great house. The settlement flourished from A.D. 1050–1150, at which time it was one of the largest Puebloan settlements in the Southwest, strategically situated between Mesa Verde to the north and Chaco Canyon to the south. Culturally it is considered a Chacoan outlier, at the northern terminus of one of the prehistoric roads that emanated from Chaco. Later occupants (in the 1200s) are thought to have had closer ties to Mesa Verde.

One of the earliest written eyewitness accounts of Aztec Ruins was provided by geologist John Newberry in 1859, who reported at that time that the walls stood 25 feet high. Both the ruins and the setting are spectacular, but the park is perhaps best known for the reconstructed Great Kiva, which was excavated in 1921 and reconstructed by Earl Morris in the 1930s. It is the only restored great kiva in the Southwest and is accessible to visitors; stepping inside provides a unique glimpse of what these ceremonial structures might have been like when they were intact. The park is now a World Heritage Site.

Location

Regional Setting

The monument is located within the San Juan Basin, a structural depression that formed during Laramide time. The basin is bounded on the north by the San Juan uplift and the San Juan volcanic field, whose remnants may be seen today in the San Juan Mountains of southwestern Colorado. To the east it is bounded by the Nacimiento uplift. To the south lie the Mount Taylor volcanic field and the Zuni uplift; to the west lies the Defiance uplift. The Hogback monocline, visible on US 64 between Shiprock and Farmington, forms the northwest boundary of the San Juan Basin.

The Rock Record

An enormous thickness of sediments (close to 15,000 feet in places) is preserved in the San Juan Basin. This thick accumulation of sediments ranges in age from Pennsylvanian to Tertiary; these are underlain by Precambrian crystalline rocks (1,400–1,750 million years old). As is the case in structural basins, the youngest strata are exposed near the center, and it is here that the thickest accumulations of sediment are found. Progressively older strata are exposed as one approaches the edge of the basin. The oldest strata and the Precambrian crystalline rocks that underlie them are exposed in uplifts along the edge of the basin—the Nacimiento and Zuni uplifts in New Mexico and the San Juan uplift in Colorado.

The bedrock within the monument is entirely Paleocene Nacimiento Formation (65 million years old). These sediments are primarily nonmarine continental sandstones and shales. They represent floodplain, river, swamp, and lake deposits. The sediments were carried into the basin from the San Juan uplift to the north and the Brazos-Sangre de Cristo uplift to the east. Outside the monument there are exposures of the younger, largely fluvial San Jose Formation, which was deposited on top of the Nacimiento.

There is a single outcrop of Nacimiento sandstone in the park. Blocks of Nacimiento sandstone are incorporated into the ruins. Other materials were used as well (the limestone disks found in the Great Kiva, for instance), some of which must have been transported some distance by the builders. Elsewhere in the San Juan Basin, the Nacimiento Formation has yielded a wealth of vertebrate fossils, as well as petrified wood, but no fossils have been found in the Nacimiento within the national monument to date.

The ruins themselves sit on Quaternary alluvial terrace deposits of the Animas River. The complex recent history of the river valley is recorded in these terraces and reworked alluvium, but the details are difficult to decipher with the untrained eye.

Geologic History

The San Juan Basin is primarily a Laramide feature (Late Cretaceous to Eocene). Prior to the Laramide orogeny, the San Juan Basin occupied a small part of the Western Interior Basin, at the western edge of the Western Interior Seaway. During the Late Cretaceous and on into the Tertiary, the basin subsided, and thousands of feet of additional sediment accumulated. These sediments were deposited in an array of marginal marine, non-marine, and terrestrial environments and include coal deposits and petroleum source rocks. The Late Cretaceous strata in the San Juan Basin have proven to be a rich source of oil and gas, coal, and coalbed methane. These strata represent a transition from the older, primarily coastal plain and marine sediments (deposited on the edge of the Western Interior Seaway) and the nonmarine strata that dominate the Tertiary record here.

The end of the Cretaceous, here and elsewhere throughout the world, is marked by a major extinction. The creatures that survived into the Paleocene inhabited a world of new possibilities, new ecologic niches. Dinosaurs are notably absent from these early Paleocene terrestrial faunas. For the first time, mammals became the dominant terrestrial vertebrates. These mammals evolved rapidly and diversified throughout the Tertiary. The Nacimiento Formation has produced fossil vertebrates, giving us an important glimpse of the world as it existed soon after the cataclysmic meteorite impact that occurred at the Cretaceous/Tertiary boundary.

The region was further affected by an episode of mid-Tertiary volcanism 35–18 million years ago, with the eruption of the San Juan volcanic field to the north. In fact, mid-Tertiary volcanism was widespread and affected much of the Southwest, including enormous portions of southwest New Mexico and Mexico. The episode of volcanism in the San Juan volcanic field began with the eruption of a series of lava flows. Later eruptions produced widespread, thick blankets of hot volcanic ash (ash-flow tufts or ignimbrites) that likely extended far enough south to cover the Aztec area, but has since been eroded away.

The Animas Valley was the largest glaciated valley on the margin of the Colorado Plateau. Late Pleistocene glaciers covered about 20 percent of the Animas River basin and about half of the river’s length. During the Pleistocene the upper Animas Valley contained one of the largest glaciers in the Rocky Mountains in the U.S., the Animas glacier. This was one of many glaciers that issued from an ice field in the San Juan Mountains. The Animas Valley contains moraines of middle and late Pleistocene age that have been identified as far south as Durango. In the vicinity of the national monument, three distinct river terraces have been identified. Such terraces provide insight into Quaternary glacial-interglacial cycles. The ruins are situated on outwash from the Animas glacier, since reworked into terrace deposits by the Animas River.

Geologic Features

The Animas river is one of the primary reasons that the prehistoric settlement at Aztec was situated here. The river runs along the eastern boundary of the park and provides a (nearly) perennial source of water, as it must have during the eleventh and twelfth century. This source of water allowed for the development of these fertile bottomlands during prehistoric times. The first European visitors to the area reported seeing signs of prehistoric irrigation features.

The San Juan basin is one of the richest and most productive oil and gas provinces in North America and has been producing since the 1920s. The monument is on the edge of the Blanco Mesaverde gas pool. There are currently three gas wells operating within the boundaries of the park, producing from Cretaceous sandstones at a depth of 4,300 feet below the surface. The San Juan Basin of New Mexico and Colorado is also one of the most prolific coalbed methane-producing areas in the U.S.

Additional Reading

Aztec Ruins on the Animas: Excavated, Preserved, and Interpreted by Robert H. Lister and Florence C. Lister. Revised edition, Western National Parks Association, 1996.

The House of the Great Kiva at the Aztec Ruin by Earl H. Morris. Originally published in 1921, reissued by Western National Parks Association,1996.