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Mima Mounds in Rolette County

Are you familiar with mima mounds? They are low circular mounds of soil on some landscapes in Oregon, Idaho, and other areas of the United States, mostly west of the Mississippi River. Mima mounds, or something very similar, have been observed in Rolette County. They are mentioned as “freckled areas” in North Dakota Geological Survey Bulletin 58, Geology of Rolette County, written by Dwight Deal (1971).

Although very difficult to identify on land, Deal noticed easily recognizable “freckled” areas on aerial photographs of some parts of the county. They consisted of circular areas between 30 and 60 feet in diameter, a few inches high, and soil a bit lighter colored than the surrounding soil, perhaps due to increased calcium carbonates brought up from lower in the profile. Similar features have also been observed in Burke County.

Mima mounds are similar features and have been studied in the Waubon Prairie Research Area in northwestern Minnesota and are thought to be the hibernating sites of Manitoba toads. One study found 3276 toads in one large mound.

A subsequent study of these mounds found that they may be associated with excavation activities of other animals as well, including pocket gophers, ground squirrels, and badgers. These activities alter the soil properties resulting in loss of soil structure, a lower bulk density, and increased permeability. Although the term mima mounds was not used in association with the freckled areas in Rolette County, they may be the same, or a similar phenomenon.

Out of curiosity I checked the southeastern part of Rolette County on Google Earth. Sure enough freckled areas were quite noticeable in the southwest corner of the county.

I have not found a definitive explanation for the freckled areas of Rolette County. But whatever the origin, it is always interesting to learn about new features of the landscape and how they may have been formed. You might want to check these interesting features for yourself on Google Earth. One freckled area is quite pronounced in Section 31, T. 159 N., R. 73 W. which is a couple miles north and east of Barton.

Chuck Lura has a broad knowledge of "Natural North Dakota"and loves sharing that knowledge with others. Since 2005, Chuck has written a weekly column, “Naturalist at Large,” for the Lake Metigoshe Mirror, and his “The Naturalist” columns appear in several other weekly North Dakota newspapers.
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