Fort Union National Monument, in New Mexico, was set up in 1954 to protect the remains and setting of Fort Union and to tell of its part in westward expansion in the Southwest. Active from 1851 to 1891, the fort was once the largest military post in the region, and its story is bound up with the Santa Fe Trail, the vital overland trade route between Missouri and Santa Fe.
The monument has two units. The smaller, western one holds the foundations of the first Fort Union and the stabilised ruins of the third fort's arsenal. The main unit holds the star-shaped second fort and what remains of the third.
The setting
Fort Union stands on the High Plains of northern New Mexico, close to where the plains meet the Sangre de Cristo Mountains to the west. At 721 acres the monument is small, but the landscape around it is geologically varied, and that variety is part of the historic site's character.
Almost the whole monument sits on the Graneros Shale, a dark grey rock laid down in the shallow sea of the Western Interior Seaway about 100 million years ago. Its thickness here is unknown but is estimated at about 150 feet (45 m). Beneath it lies the Dakota Sandstone, of about the same age, which records a shift from coastal plain to shallow sea as the seaway advanced over the land. It reaches up to 200 feet (60 m) thick near the fort and crops out just west of the smaller unit, where the Fort Union fault brings the two formations side by side.
The fort's depot warehouses, with the Sangre de Cristo Mountains far behind. NPS photo.
A mesa and mountains made by magma
Two landmarks in the fort's view, Black Mesa and the Turkey Mountains, have a very different history from the ground inside the monument: both are the work of igneous processes.
The Turkey Mountains, to the northeast, are a laccolith: a shallow igneous intrusion in which a dome-shaped body of magma with a flat floor pushed up the sedimentary layers above it. When it was emplaced is not well known, but it was sometime in the middle to late Tertiary. Older sedimentary rocks are exposed in these mountains and in the foothills of the Sangre de Cristo above the fort.
Fort Union lies near the southern end of the Ocaté volcanic field, active between 8.3 and 0.8 million years ago and one of several young volcanic fields lined up along the Jemez Lineament in New Mexico. It has a number of vents, some of which built cinder cones, and wide lava flows, mostly basalt.
Black Mesa is capped by one of the older flows and is a textbook case of inverted topography. When the lava erupted, it ran into low ground. Being harder and more resistant to erosion than the sedimentary rock around it, it was left standing as erosion wore the surroundings away, so what were once valleys filled with lava are now high ground.
The Turkey Mountains, a laccolith, fill the northern horizon at Fort Union. NPS photo by Katie KellerLynn, Colorado State University.
What the soldiers built with
The fort was built from local geology. Dakota Sandstone was quarried for building stone, and clay from the Graneros Shale went into adobe and mortar.
Wind, water and wagon ruts
Wind is a constant on the High Plains, and dust storms were frequent while the fort was in use. The ground around it was trampled and grazed, at times stripped bare of plants and open to the wind. Windblown deposits were reported then, but none appear on today's geologic map: the land has largely grown back over, which has all but stopped erosion by wind.
Wolf Creek, a spring-fed stream that flows all year, runs between the two units. Alluvium covers its channel and floodplain, but no older terrace deposits have been mapped along it.
The ruts left by wagon wheels and animals began as disturbed ground but are now among the most important cultural resources of Fort Union and the Santa Fe Trail — some of the trail's main surviving physical traces. Most of the ruts at the fort are stable and are not being cut deeper.
Hazards
Geological processes continue around the monument over seconds to years, and visitors should stay alert. There is some rockfall, especially along the Dakota Sandstone ridge on the west side of the smaller unit; only a short stretch of the ridgeline crosses the boundary, but the monument lies downslope of it. Flash floods are a possible hazard along Wolf Creek. The nearest faults that have moved in the Quaternary, the El Oro and Quebraditas faults, are at least 30 miles to the west.
No caves, no fossils — so far
There are no caves or karst features in the monument. Basalt flows sometimes contain lava tubes, but none have been recorded in the nearby basalts of Black Mesa and Wolf Creek.
No fossils have been found in the monument or close by. Elsewhere in New Mexico the Graneros Shale has yielded oysters, ammonites, snails and foraminifera, and the Dakota Sandstone elsewhere in the northeast of the state has dinosaur tracks and petrified wood. Fossils, most likely molluscs, could also turn up in the young alluvium along Wolf Creek, and blocks of Dakota Sandstone — building stone among them — can move downslope into the monument from the ridge to the west. Fossils in national parks are protected by the Paleontological Resources Preservation Act of 2009.
Fort Union belongs to the Great Plains physiographic province and shares its geological history and several of its rock formations.
The Geologic Resources Inventory's overview map of Fort Union National Monument, with Black Mesa at its centre. NPS.
Maps and reports
The NPS Geologic Resources Inventory, which produces digital geologic maps and reports for more than 270 parks, has published a report and digital geologic map data for Fort Union National Monument. A soil survey for the monument has also been completed. Both are on the NPS DataStore.
This page is part of the NPS Geodiversity Atlas, which gathers information on the geology and soils of each park — rocks, minerals, sediments, fossils, landforms and the processes that shape them — for education, geoconservation and park management.
Sources
- National Park Service, "NPS Geodiversity Atlas—Fort Union National Monument, New Mexico." https://www.nps.gov/articles/nps-geodiversity-atlas-fort-union-national-monument-new-mexico.htm
- NPS Geologic Resources Inventory, Fort Union National Monument products: https://irma.nps.gov/DataStore/SavedSearch/Profile/2669
- Photographs: National Park Service; Turkey Mountains by Katie KellerLynn.
Licence: CC0 1.0 (public domain) · Adapted from www.nps.gov
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