Coteau des Prairies

I poured over maps for a project that’s been slowly forming at the back of my mind — I know that must come as a surprise — and I noticed an anomaly I’d overlooked in all the other times I’d examined this particular patch. I think it had to do more with the way the map was drawn than the actual feature itself. This particular topographic relief map just happened to select 2,000 feet (610 meters) as a breakpoint between color gradients.

Magically, a thin line rose from the plains of northeastern South Dakota. It would have disappeared entirely into the background had the mapmaker chosen 2,500 feet or a half-mile to differentiate elevation; an indistinguishable element.

Elevation Gradients in South Dakota
SOURCE: U.S. Geologic Survey Topographic Relief Map, in the public domain

I’d stumbled upon the Coteau des Prairies, a 200 by 100 mile plateau located primarily in South Dakota but also extending into Minnesota and North Dakota to a lesser extent. Early French explorers provided the name, with "Coteau" translating roughly to hill or upland in this context.

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Granted it’s barely perceptible at first glance which is probably why I’d never paid much attention to it before. Look closely at Google Maps in terrain mode: notice the area marked by all the small ponds and lakes and you will begin to see it. Glaciation formed countless pockmarked depressions which now serve as natural basins. Glaciers are responsible for just about everything that one observes here. Numerous flows of ice moving back-and-forth across the landscape left deposits that reached as high as 900 feet (275 m).

Something interesting happened in the final glaciation. This time one place had just enough resistance to block the frozen mass as its tendrils descended from the north. The Coteau des Prairies plateau remained ice-free.

The flow split as it moved south , creating a recognizable tip just east of present-day Havana, North Dakota. It gouged the surrounding plains to the east and west as it passed, allowing elevation features to become even more pronounced upon retreat. Coteau des Prairies would have appeared as ice-free sawtooth cutting into the snowcap during the Wisconsinan glaciation, perhaps ten or fifteen thousand years ago.

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Google Street View doesn’t do it much justice, unfortunately. Nonetheless, pan the image above about 180° and you’ll see a wooden observation post in the distance. That’s the Joseph N. Nicollet Tower of the Heritage Museums of the Coteau des Prairies, in Sisseton, South Dakota. It’s supposed to be a great view of a tri-state area and the admission price is right (free). The tower adds another seventy five feet of height above the plains which would be a distinct advantage to Street View.

Coteau des Prairies Topography from the North
SOURCE: North Dakota State University

It’s much easier to observe the formation with an exaggerated scale. This image, looking from the north increases elevation by a factor of 20X (you can use Havana, ND in the lower-right as a point of reference). Now it’s obvious to see why Coteau des Prairies has often been described as having a "flatiron" shape. The wedge and flat plateau practically jump from the screen.

The North American continental interior is considered comparatively bland and featureless stereotypically. While the landforms may not be as dramatic as elsewhere, look closely enough and all sorts of interesting variations begin to appear.

4 Replies to “Coteau des Prairies”

  1. Random thing I love: When travelling the Mass Pike (I-90) West in western MA, there’s a huge green highway sign stating that at XX feet (I forget the specifics), that you are at the highest elevation on I-90 until you reach somewhere in South Dakota.

    I haven’t ever seen an analogous official sign like that anywhere else… and I wonder if it has a counterpart out in SD?

    1. Here’s the sign, Steve:

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      It’s been intentionally blurred by Google. A much better image can be found on this flickr photo. It reads, "Highest Turnpike Elevation 1724 feet. Next highest elevation on I-90 Oacoma, South Dakota 1729 feet." It doesn’t appear to have a counterpart in South Dakota, at least according to my quick search of the Intertubes.

  2. Nothing special about that elevation in SD. Oacoma is in the central/south central part of the state on the west bank of the Missouri River. Further west near Rapid City, I-90 skirts the Black Hills (Mt. Rushmore). My guess I-90 in SD gets close to the 4000′ range and then considerably higher in Montana as it goes through the Rockies.

    1. OTOH, the highest point of I-29 does appear to be on the Coteau des Prairies, near the rather appropriately named town of Summit, South Dakota.

      (Apparently, “Summit, South Dakota received its name because the town had an altitude greater than any other town between the Mississippi and Missouri rivers”. This might still be true, depending on what you count as “town”… no problem with “between” – it’s right on the drainage divide!)

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