Geology Sets the Stage: Hovenweep National Monument by Allyson Mathis
Towers and other prehistoric structures sitting on the Naturita Formation along the edge of a small canyon in Hovenweep National Monument
Hovenweep National Monument was proclaimed in 1923 to protect a series of ancestral Puebloan free-standing towers, villages, and other structures dating from approximately 1300 CE. The monument is considered to contain some of the finest examples of this type of prehistoric masonry, which is distinct from other Puebloan structures, including at the nearby Mesa Verde National Park.
Yet geology is also an important part of the story of Hovenweep National Monument. It influenced the people who built and occupied the towers and other structures there and the geologic landscape formed the tableau of their lives. The structures are constructed of local stone, and are perched along the edges of small canyons of minor tributaries that connect with larger ones that flow into the San Juan River. Spring and seeps provided water, and nearby deposits of a type of fine-grained windblown (eolian) sediment called loess made good soils for growing crops of corn, beans, and squash.
Located nearly 120 miles south of Moab, Hovenweep is also on the Colorado Plateau, e.g., the Four Corners region that has an overall high elevation and generally flat-lying sedimentary rocks. More specifically, Hovenweep is an area known as the Blanding Basin, where there is a subtle downwarping of the rock layers (formations to geologists). As a result of this downwarp, the rocks exposed at Hovenweep are younger in age than the majority of the rocks in the Moab area, including the cliffs of Canyonlands National Park and above Moab and the rocks that hold the arches of the other national park in southeastern Utah. Close up of Hovenweep architecture. The building stone is likely from the Naturita Formation.
Almost all of the rocks exposed in Hovenweep are from only two different layers and they are both Cretaceous in age, versus the slightly older Triassic and Jurassic rocks that dominate the Moab area. At Hovenweep, the Naturita (or Dakota) Sandstone is found on the benches and the upper part of the monument’s small canyons, and the older Burro Canyon Formation is exposed in the canyons.
The Burro Canyon Formation is equivalent to the Cedar Mountain Formation found north and west of Arches National Park and at the Mill Canyon Dinosaur Tracksite near Moab, with the difference between the two layers being largely one of nomenclature. By convention, these rocks are known as the Cedar Mountain Formation west of the Colorado River and the Burro Canyon to the east (although, of course, the Colorado River didn’t exist more than 100 million years ago when they were deposited. Coal in the Burro Canyon Formation in Hovenweep National Monument.
The Burro Canyon includes sediments that were deposited in rivers and on their floodplains, and contains pebble-rich sandstone (conglomerate), fine-grained sediments, and even minor deposits of low-grade coal.
The Naturita Formation is a widespread but thin sandstone layer found through southeastern Utah and western Colorado. This layer formed along the shoreline along of a vast inland sea known as the Western Interior Seaway. The Western Interior Seaway stretched from the Arctic to the Gulf of Mexico during the late Cretaceous Period, a time of high sea level.
This particular sandstone layer in Hovenweep and elsewhere in southeastern Utah was until recently known as the Dakota Sandstone, but it has been recently reclassified as the Naturita Formation to adhere to the naming conventions that geologists use for rock layers. A formation is named after the place was first scientifically described and outcrops found elsewhere must form a continuous unit with that original (or type) area. The Dakota Sandstone was first recognized on the east side of the Western Interior Seaway so it is not possible that rocks on the west side can be continuous with the type section on the opposite shore. The Naturita Formation is named after small town in western Colorado, so exposures at Hovenweep easily correlate to that location. Sleeping Ute Mountain (in the background) in western Colorado about 18 miles east of Hoveenweep. Sleeping Ute Mountain has the same general origin as the La Sal Mountains near Moab. Both ranges were formed from a type of shallow igneous intrusion where magma (molten rock material) was injected to close to the surface, but solidified without erupting. Both mountains, as well as the Abajo and Henry Mountains, formed less than 30 million years ago.
The redesignation of these rocks to the Naturita Formation is so recent that many geologic maps and existing geology publications have not been updated yet to reflect this change, and even when they have, they often refer to the old nomenclature since it is so well known.
Although Hovenweep doesn’t have the flashy geology of a Canyonlands or Arches, its geology remains essential and the people who once lived in the area. If for no other reasons, because it is likely that the people who built the towers of Hovenweep may have appreciated the Naturita as a source of as good building stone.
A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab.
To learn more about Moab’s geology, visit the Geology Happenings archive online at https://www.moabhappenings.com/Archives/000archiveindex.htm#geology
A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab.
