Aztec Ruins' Geology: A Great House Built From a Glacier's Leftovers

Aztec Ruins National Monument, in the city of Aztec, New Mexico, preserves an outstanding ancestral Puebloan farming community — and its builders drew on the ground beneath them. The cobbles, gravel and sand in its walls came from the river beside it. Its mortar was made from windblown dust. And the whole settlement stands on debris washed out of an Ice Age glacier.

Despite its name, Aztec Indians never lived here. Between Chaco Canyon to the south and Mesa Verde to the north, Aztec was one of the largest ancestral Puebloan settlements in the Southwest. As the park's brochure puts it, "By the time building ceased in the late 1200s, the complex consisted of several great houses, tri-walled kivas, small residential pueblos, earthworks, roads, and great kivas." Proclaimed in 1923, the monument now covers 318 acres (129 ha).

The river

The builders used the year-round water of the Animas River, along the monument's eastern edge; many modern Puebloan people know the site as the "Place by Flowing Waters." The Animas runs from the San Juan Mountains through the San Juan Basin, crossing many kinds of rock before joining the San Juan River near Farmington — which is why its deposits at the monument range so widely in rock type and grain size, from cobbles to loose sand. Much of that alluvium went into the buildings.

The glacier

The Animas valley once held the largest glacier bordering the Colorado Plateau, one of several valley glaciers flowing from a small ice field in the San Juan Mountains. Aztec Ruins sits on its outwash, perhaps reworked between glaciations. Glacial end moraines stand at Durango, Colorado, in three groups from about 15,000 to 360,000 years old.

The ancestral river probably formed after the Laramide Orogeny, the mountain-building that began about 70 million years ago — perhaps as late as the Miocene — and began cutting its valley between 18 and 3 million years ago, before the glaciers. It is still cutting down, and its pace has likely quickened toward the present.

Steps in the valley

When a river cuts into its floodplain, the old floodplain is left behind as a flat step: a terrace. About 34 terrace levels have been identified in the Animas valley between Durango and Farmington, rising as high as 2,165 feet (660 m) above the valley floor; three, cut into bedrock, lie within the monument. Geologist Gillam dated them in part using the Lava Creek B ash — about 640,000 years old, from an eruption at Yellowstone. Terraces matter because they can record whole glacial-interglacial cycles, where moraines mark only a glacier's farthest reach.

Geologic map of the monument and surroundings, from the Geologic Resources Inventory. NPS.

Dust, clay and fossils

• Loess — windblown dust up to 3 feet (0.9 m) thick — mattered to the builders: they dug underground structures into it, made mortar from it, and its clay held moisture for farming.
• The bedrock is the Nacimiento Formation: gray, green and purple claystone, shale and siltstone, and gray and yellow sandstone, used in construction — green claystone makes a distinctive band in one wall.
• The one outcrop of it in the monument makes finding fossils in place unlikely, though petrified wood turns up in terrace gravels. Fossils in the park's museum collection came from archeological digs and may have been gathered by ancient people elsewhere; all NPS fossils are protected under the Paleontological Resources Preservation Act of 2009.

Local Nacimiento Formation stone in a wall, with a band of green claystone. NPS.

Sources

Based on "NPS Geodiversity Atlas — Aztec Ruins National Monument, New Mexico," National Park Service, Geologic Resources Inventory, drawing on Gillam (1998) and Thybony (1992); a work of the United States government in the public domain. The source gives the length of occupation both as "about two centuries" and as "ca. AD 1100–1130," so neither is repeated here.