NDGS - North Dakota Notes #16

NORTH DAKOTA NOTES NO. 16

DUNES
by John Bluemle

    No, this is not a sequel to Frank Herbert's classic science fiction novel Dune. However, some people do have a hard time taking me seriously when I start telling them about North Dakota's sand dunes; perhaps they think I'm talking fiction. You find dunes in the desert, right? Yes, you may, but you also find them in North Dakota. North Dakota has several areas of fairly good-sized sand dunes. None remotely like the ones described by Herbert on the planet Dune, or even the more earthly dunes found near Alamosa, Colorado in Great Sand Dunes National Monument, or the widespread dunes in the sand hills of western Nebraska. Even so, our dunes are interesting features and a pleasant place to spend an afternoon roaming around (but best on a clear summer day; dunes are about the easiest place I know of to get lost in during a snowstorm or on a foggy day). The dunes in the Sheyenne National Grasslands are particularly interesting for their extensive areas of native prairie.

    All of the larger areas of dunes in North Dakota are found in areas where sand was originally deposited by glacial-related processes. Most of the dunes in North Dakota probably formed between about 8,500 and 5,500 years ago. They formed during a period of time that geologists refer to as the "Altithermal Interval." The Altithermal Interval ("Altithermal" = maximum temperature) was the warmest and driest time since the end of the Ice Age, a time when most of the world experienced mean annual temperatures higher than those today (one of the numerous episodes of "global warming" we’ve experienced over the millennia). During much of the long, warm and dry Altithermal Interval, conditions were similar to those during the 1930's in North Dakota, when the cover of vegetation became so sparse it was no longer effective in controlling wind erosion. Many North Dakotans still remember the terrible dust storms over the plains during the so-called "dirty thirties," and of course the drought of the late 1980's and early 1990's is still fresh in most people's minds.

Dunes near Denbigh, McHenry County (Photo by J. Bluemle).

Dunes at Denbigh, McHenry County (Photo by J. Bluemle).

    Most of North Dakota's sand dunes are found in places where sand was deposited by rivers flowing into lakes at the end of the Ice Age. Some of the more interesting areas of dunes in North Dakota are those south of Walhalla in Pembina County, where the Pembina River entered Lake Agassiz; near Wyndmere and McLeod in northeastern Richland and eastern Ransom counties, where the Sheyenne River flowed into glacial Lake Agassiz; southeast of Oakes in southwestern Sargent County where the James River flowed into glacial Lake Dakota; and in McHenry County near Denbigh and Towner, where the Souris River flowed into glacial Lake Souris.

    In places where sediment-laden, Ice-Age rivers flowed into glacier-dammed lakes like Lake Agassiz and Lake Souris about 10,000 years ago, they deposited large amounts of well-sorted sand. The sand built up fans at the mouths of the rivers. The heavy, sediment-laden river water stayed near the bottom of the lake and spread out over a broad area. The coarser materials had either been deposited farther upstream, on the river floor itself, or they were deposited very close to the river mouth; only the finer sand and coarse silt made it into the lake. It was this well-sorted sand that was later blown into dunes.

    Many of our North Dakota dunes are from 50 to 80 feet high and a few are nearly 100 feet high. It's hard to visualize by standing on the top of a dune (you need to be higher, in an airplane), but many of the dunes have a strong northwest-southeast linearity, parallel to the direction the wind was blowing when it built them. These are known as longitudinal dunes. A few of our dunes are crescent shaped with their convex side pointing toward the northwest or nearly north (upwind), and their ends pointing southeastward (downwind). In North Dakota, the crescent-shaped dunes are not as large as the longitudinal dunes. Geologists generally refer to them as barchan dunes (pronounced bar-con; the word is Turkish and means "sand hill"). They are also known as horseshoe dunes or parabolic dunes.

Wind blown ripples southwest of Walcott, North Dakota (Photo by J. Bluemle).

    Sand dunes are certainly our most obvious and spectacular wind-blown landforms, but they are not the only kind of deposit in North Dakota that was formed by the wind. While the wind was shaping the sand into dunes, much finer, silt-sized particles were being blown greater distances and spreading out over much wider areas. In fact, much of North Dakota is veneered by a discontinuous layer of wind-blown silt that geologists call "loess" (rhymes with "puss;" the word translates approximately "loose" from German). In places along the Missouri River valley in northern Mercer County; just south of Garrison Dam in McLean County; and in parts of Emmons County along the Oahe Reservoir the loess is as much as 20 feet thick. Over much of southwestern North Dakota, the loess cover is from 3 to 6 feet thick.

Dunes nortwest of Fort Yates in the Porcupine Creek Valley.

    Generally, the relatively flat areas of wind-blown loess, which are essentially the deposits of Altithermal dust storms, occur downwind from river flood-plain deposits or downwind from any place the wind could sweep over a broad area of fine-grained sediment. In parts of western Iowa, just east of the Missouri River Valley, deposits of loess more than 100 feet thick are found, and in the Gobi Desert in Mongolia the wind-blown silt may be as much as 1,000 feet thick. In North Dakota and other plains states during the 1930's, as much as several feet of wind-blown soil collected along fence rows and on buildings; we've all seen pictures of the dust-bowl conditions during the thirties.

Dunes and aspen near Hamar, Eddy County, North Dakota (Photo by J. Bluemle).

    The next nice summer day that you happen to be traveling on Highway 2 in the area between Towner and Denbigh, or along Highway 27 east of Lisbon, or in any of a number of places where dunes are easily accessible, stop for a while and walk out into them. The dunes near McLeod in eastern Ransom County are part of the Sheyenne National Grassland and a beautiful place to see undisturbed native prairie. The dunes in the Walhalla area are special too as they are the only ones in North Dakota that are largely wooded. Other smaller areas of dunes may be found near Hamar in eastern Eddy County and near Stanton on both sides of the Missouri River in Mercer and McLean counties. Wherever you stop among our dunes, I think you'll enjoy them as much as I do.

06-Aug-2007>
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