Geology

Geologic map of Fort Union area shows the large mesa of Dakota Sandstone in light orange on the left side and the igneous deposits of basalt in pink from the nearby Ocate volcanic field. The black line along the right side of the Dakota sandstone represents the fault that raised the mesa above the level of the surrounding plains.

R.B. Johnson, U.S.Geological Survey

 

Geology plays a prominent role in the visual landscape of Fort Union. Black Mesa to the west was created by a fault that raised the block of Dakota Sandstone several hundred feet above the surrounding plain. The mesa is topped by basalt from the Ocaté volcanic field to the north, and the extinct volcano Cerro Pelon is visible from the fort. The Turkey Mountains to the east were produced by a large igneous intrusion, or uplift.The Graneros Shale, laid down beneath an ancient sea, lies beneath the fort area and provided soil for the adobe used to the build the third Fort Union.

 

Cerro Pelon, an extinct volcano, on the right side of the photo as seen from Fort Union.

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Volcanic Features

Cerro Pelon, the extinct volcano, graces the northern horizon beyond Fort Union as the most prominent feature of the nearby Ocaté volcanic field. Active for seven million years, the Ocate volcanic field is part of the Jemez Lineament, a chain of volcanic fields extending for several hundred miles across New Mexico from northeast to southwest. The Ocaté volcanic field has been quiet for the last 800,000 years. It also produced the basalt that caps Black Mesa to the west and the Turkey Mountains to the east.

 

Black Mesa dominates the terrain west of Fort Union, with the Rocky Mountains looming in the distance.

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Black Mesa

Black Mesa, marked in the photo by the dark band above the ruins of Fort Union, was created by a fault that lifted the mesa several hundred feet above the plains in the foreground. Black Mesa is composed of Dakota Sandstone, a rock that formed on the beach of a large inland sea almost 100 million years ago. An aquifer in the Dakota Sandstone supplied the fort with water. The mesa is capped by hard basalt from the Ocaté volcanic field. The snow-capped Rocky Mountains in the distance show how close Fort Union is to the edge of the great prairie.

 

The Turkey Mountains are just a few miles east of Fort Union.

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Turkey Mountains

Formed about 15 million years ago, the Turkey Mountains provided the main wood supply for Fort Union. Dozens of wagons lumbered across the plains from the fort, making the short trip to the Turkeys, where soldiers on fatigue duty set up lumber camps. Technically, the Turkeys are a laccolith. A laccolith is a mushroom-shaped body of magma that pierces the Earth's crust, splitting the overlying rock layers into the shape of a dome.

 

Fort Union sidewalk, made from Dakota Sandstone, showing the ripple marks created by water currents in the original sand sediments.

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Building Materials

Most of Fort Union was built with geologic materials. Dakota Sandstone was quarried out of a canyon in Black Mesa and was used for the fort prison, building foundations and sidewalks. Soil for the many adobe bricks used to construct the buildings came from fields to the north of the fort. The soil was produced from the Graneros Shale that underlies the entire fort. Greenhorn Limestone, which crops out above the surface in a few places near the fort, was used in the lime kilns that manufactured lime for plaster and mortar. All three rock layers--Dakota Sandstone, Graneros Shale and Greenhorn Limestone--were produced over a span of about five million years when a large inland sea covered the western states. Different types of rocks were formed as water depth, temperature and currents changed over time.