Near Garrison in McLean County, North Dakota — The American Midwest (Upper Plains)

Geology

Indian Hills

Photographed by Barry Swackhamer

1. Geology Marker

Captions: (left side, top to bottom) (unable to read); Colorful reddish layers of "clinker" or scoria.; Layers of windblown ash and sediments deposited by ancient rivers turned over time into sandstone, coal, siltstone, and mudstone layers.

Inscription. The Missouri River winds through the fertile and alluvial floodplains within the river trench.
Overlooking the valley is the drift prairie, covered by glacial till, which is equally fertile. Within the river are found bars of sand , silt, and clay, which has been transported and deposited by the river waters. During the last Ice Abe, which ended about 10,000 years ago, melting glaciers forced great amounts of water down the valley, moving and depositing coarse sand and gravel. Terrace deposits in the modern valley show lenses of some of this sand and gravel. During times of low water on Lake Sakakawea, the feet of the steep banks and the shoreline are good places to hike and explore. Chunks of coal, fossilized tree remnants, concretions, clinker, and glacial rocks can be found. At Beulah Bay, on the south side of the lake, there are fine exposures of the Sentinel Butte formation which contain many petrified logs and stumps. Some of the petrified trees are extremely well preserved. The US Army Corps of Engineers, which manages the shoreline of Lake Sakakawea, prohibits the collection of artifacts, rocks, or fossils without

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a permit. Lignite, a low grade, immature coal, shows up in several stratigraphic locations in the Sentinel Butte formation in beds usually 3 to 5 feet thick. A few miles southeast of Garrison, in what is now a wildlife management area, is the abandoned Custer Mine, operated during 1946-1964 to provide fuels for a lignite=burning power plant owned by the Otter Tail Power Co. at Washburn. The coal was transported from mine to plant by railroad cars. The property was deeded to the North Dakota Game and Fish Department in 1984 and partially resolved during the 1990s. Occasionally, over the centuries, lignite beds have been fired by lightning strikes, prairie fires, or spontaneous combustion and have burned for considerable periods of time. One of the results is the baking of adjacent clay strata resulting in the formation of clinkers which carry in color (black to yellow to red) and hardness (friable to fused). Red fused clinkers, known locally as scoria, is often mined for road-surfacing material for use on building roofs.
Also to be seen are a number of buttes, formed by erosion and surviving caprice, and noticeable for the many

Photographed by Barry Swackhamer

2. Geology Marker

strata varying both in thickness and color, Some distance to the west, the Blue Buttes cover about 50 square miles and are capped by sandstone of the upper member of the Golden Valley formation. Bright white kaolinite clays, and lower member of the Golden Valley formation, crop out along the sides of some of these buttes.

Take Nothing but Pictures, Leave no Trace

Erected by Indian Hills Resort.

Topics. This historical marker is listed in this topic list: Natural Features.

Location. 47° 36.045′ N, 102° 5.768′ W. Marker is near Garrison, North Dakota, in McLean County. It can be reached from the intersection of 73rd Avenue NorthWest and 14th Street NorthWest. The marker is located on the Nux Baa Ga (meaning “All the People” in Arikara) Trail. Touch for map. Marker is in this post office area: Garrison ND 58540, United States of America. Touch for directions.

Regionally, this marker is in North Dakota’s Mandan, Hidatsa, and Arikara Nation,

in the Standing Rock Sioux Tribe,

in the Drift Prairie,

and on the Missouri Plateau.

It is also in the American Lewis & Clark Corridor,

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on the prairies,

and on the Northern Plains.

Globally, it is in North America,

the Western Hemisphere,

the Western World,

and the Anglosphere.

Historically, it finds itself in what was once Rupert’s Land

and also the Louisiana Purchase.

Other nearby markers. At least 8 other markers are within walking distance of this marker: Nightwalker's Butte (about 600 feet away, measured in a direct line); a different marker also named Nightwalker's Butte (about 600 feet away); Lewis & Clark (approx. 0.2 miles away); Sakakawea (approx. 0.2 miles away); Lake Sakakawea (approx. 0.2 miles away); Four Bears Bridge (approx. 0.2 miles away); Contemporary Towns (approx. 0.2 miles away); Encampments (approx. 0.2 miles away). Touch for a list and map of all markers in Garrison.

Credits. This page was last revised on September 24, 2025. It was originally submitted on September 24, 2025, by Barry Swackhamer of Brentwood, California. This page has been viewed 64 times since then and 31 times this year. Photos: 1, 2. submitted on September 24, 2025, by Barry Swackhamer of Brentwood, California.