Fort Union National Monument

Fort Union was established in 1851 to provide a much-needed military presence to travelers on the Santa Fe Trail. The trail had been active since 1821, when much of what is now New Mexico became U.S. territory in 1848, at the end of the Mexican War. Fort Union’s spectacular location on the western edge of the Great Plains was a strategic one, sited as it was near the junction of the Mountain Branch of the Santa Fe Trail with the Cimarron Cutoff. Between 1851 and 1891 Fort Union contained the largest American military presence in the Southwest; over 1,600 troops were stationed there in 1861. In 1878 the railroad came to New Mexico over Raton Pass, and by 1891 Fort Union had been abandoned. Visitors to the national monument can see some impressive historic ruins, including the foundations and adobe walls of many of the original buildings, and remnants of the deeply-rutted Santa Fe Trail.

Location

Regional Setting

Fort Union National Monument sits at the western edge of the Great Plains, in the shadow of the southern Rocky Mountains. The mountains to the west are the Sangre de Cristos. The Pecos River, which originates in the Sangre de Cristo Mountains, is not far to the west-southwest. This high, arid portion of the Great Plains west of the hundredth meridian is often referred to as the High Plains.

Fort Union also lies near the southern edge of the Ocaté volcanic field, which covers portions of the southern Rocky Mountains and the Great Plains. The cinder cones and lavas associated with this field are a prominent part of the landscape in the vicinity of Fort Union. The eruptions associated with the Ocaté volcanic field began a little over 8 million years ago and continued for more than 7 million years. The youngest flows are approximately 800,000 years old.

The Rock Record

To the north, in the Raton Basin, the High Plains are mantled with thick accumulations of Tertiary sedimentary rocks. However, in this part of the High Plains, erosion has removed most of these, exposing Cretaceous marine sedimentary rocks. In many places these marine sediments are covered with late Tertiary volcanic rocks. The monument itself sits on Upper Cretaceous Graneros Shale. The cliff at the turnoff from Exit 366 on I–25 to the monument (via NM 161) exposes outcrops of the Cretaceous Dakota Group. Dakota Group sandstones are exposed on the west side of NM 161 between I–25 and the monument. Black Mesa, to the northwest, is capped with over a hundred feet of basalt from the Ocaté volcanic field.

Geologic History

During the Late Cretaceous, much of the North American interior was occupied by a vast, shallow, inland sea that stretched from what is now northern Canada to Mexico. The Western Interior Seaway, as it has come to be known, prevailed for 30 million years, reaching its maximum extent about 90 million years ago. The Dakota Group was deposited as these seas first advanced from the north in the Late Cretaceous, in response (at least partly) to a worldwide rise in sea level 100 million years ago. The slightly younger Graneros Shale (here the basal unit of the Mancos Group) represents muds that were deposited in slightly deeper waters as the strand line migrated south-southwest. The precise age of such deposits obviously varies somewhat from place to place, and although we speak of regional transgressions as if they were single events, they in fact consisted of shorelines that likely migrated back and forth through time, as least locally, giving us complex, interbedded lithologies that are sometimes difficult to unravel.

The Laramide orogeny 75 million years ago marked the birth of the Rocky Mountains (including the Sangre de Cristos) and the end of the Western Interior Seaway toward the end of the Cretaceous. The Laramide orogeny, which lasted some 30 million years, was responsible for much of the structure and shape of this part of North America, including some of the broad, regional uplift associated with the High Plains. For the remainder of the Tertiary, this part of the continent was above sea level.

With the onset of the Laramide orogeny, enormous quantities of sediment were shed eastward from the rising Rocky Mountains and accumulated to great thickness on what is now the Raton Basin. After the Laramide, the plains saw alternating episodes of deposition and erosion, with streams from the mountains to the west shedding debris onto the plains in the east. In the late Cenozoic, streams draining the eastern flank of the Sangre de Cristos deposited an extensive deposit of sediment that includes the Ogallala Formation, the High Plains aquifer that today provides an enormous amount of water to the region, primarily for irrigation. This deposition was followed by another episode of uplift, and much of the elevation we now associate with the High Plains is the result of regional uplift that has occurred only in the past 5 to 10 million years. It is this most recent rejuvenation of regional uplift that has caused the streams in this area, including the Pecos and the Canadian Rivers, to incise their channels into this thick sedimentary cover.

The Ocaté volcanic field is one of a series of volcanic fields in northern New Mexico that appear to be part of a regional trend of Late Cenozoic volcanic activity that occurred in New Mexico over the past 10 million years. That regional trend, known as the Jemez lineament, stretches from the Raton/Clayton volcanic field on the east, through the Valles caldera, Mount Taylor, and on into the Zuni–Bandera volcanic field on the west, extending ultimately into Arizona. The volcanic features in the immediate vicinity of Fort Union are all part of the Ocaté volcanic field, which was active from 8 million years ago until just under a million years ago.

Geologic Features

Volcanic Features — The lava-capped mesa to the northwest is Black Mesa. The volcanic deposits on top of the mesa are Pliocene basalts from the Ocaté volcanic field. Maxson Crater, due east of the monument, is the source of the lava flow that follows the course of the Mora River to its confluence with the Canadian River (see map). The flows associated with Maxson Crater are young (about 1.6 million years old).

The Turkey Mountains — Visible on the horizon to the northeast are the Turkey Mountains. The uplift associated with the Turkey Mountains is the result of a laccolith, a relatively shallow igneous intrusion. The laccolith is not exposed, although drilling has confirmed its presence. The precise age of the intrusion is unknown, though it must be younger than the Cretaceous strata that were deformed when it was emplaced. Today the Turkey Mountains are surrounded by younger lavas of the Ocaté volcanic field.