Fort Laramie National Historic Site: A Fort on a River Terrace

From the National Park Service's Geodiversity Atlas.

Fort Laramie was built on a terrace cut into Quaternary floodplain deposits of the Laramie River, and the park lies almost entirely on mapped floodplain. Its landscape is shaped mostly by recent river processes.

Fort Laramie and the Laramie River. NPS.

The ground beneath the fort

• Bedrock of the region, though not exposed in the park: the Oligocene White River Formation, an unnamed conglomerate sequence, and the early to early-middle Miocene Arikaree Formation.
• Terrace gravels along the Laramie and North Platte rivers, of Quaternary age (2.588 million years ago to the present), are up to 6 m (20 ft) thick and record at least three stages of the rivers cutting down.
• Finer sediments and gravels come mostly from weathered local rocks of Oligocene (33.9 to 23.03 million years ago) and Miocene (23.03 to 5.332 million years ago) age; some gravel may come from the Laramie Mountains and the Hartville Uplift, which the rivers flow through.

A meandering river

The Laramie and North Platte are meandering rivers: a single channel swings back and forth across the floodplain in loops and bends. Such rivers erode sideways rather than down, wearing away loose sediment on the outside of each bend and dropping it on the inside, where the current slows.

At the fort, the Laramie River both adds land and takes it away as it cuts its banks. Erosion on both banks can increase during floods and the spring runoff, and releases from the Grayrocks Reservoir upstream may affect it too.

The river can also expose cultural resources. Erosion has uncovered bison and elk bones from historic hunting, the quartermaster's dump and remains of the quartermaster's corral.

Oil and gas

The park and Goshen County lie on the northwest edge of the Denver Basin, also called the Denver-Cheyenne or Denver-Julesburg (DJ) Basin. It formed about 75 to 35 million years ago, during the Laramide Orogeny, and spans parts of southeastern Wyoming, southwestern Nebraska, northwestern Kansas and northeastern Colorado. The Denver and Powder River basins both produce a great deal of oil, and oil could conceivably lie along the Denver Basin's margin near the park.

Fossils

Fossils from the late Eocene through the late Pleistocene have been found around Fort Laramie. In the late 1800s, soldiers from the fort often escorted Othniel C. Marsh, the famous Yale dinosaur hunter, on expeditions, and officers collected fossils in the area on their own as well. All fossils in national parks are protected under the Paleontological Resources Preservation Act of 2009.

Region, maps and reports

Fort Laramie belongs to the Great Plains province and shares its geologic history with a region far beyond the park. The Geologic Resources Inventory has produced a geologic map and report for the site.

The Geologic Resources Inventory's map of Fort Laramie. NPS.

Sources

• National Park Service, "NPS Geodiversity Atlas—Fort Laramie National Historic Site, Wyoming." https://www.nps.gov/articles/nps-geodiversity-atlas-fort-laramie-national-historic-site-wyoming.htm — which cites McGrew (1963).
• Rewritten in hubnx's own words.