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 vertical scale of the map 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.


A Feature Appears

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. U.S. Geologic Survey Topographic Relief Map. Public Domainl
Elevation Gradients in South Dakota

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

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 (map): notice the area marked by all the small ponds and lakes and you will begin to see it.


Glaciers and Ice Ages

Welcome to Havana, ND. Photo by Jimmy Emerson, DVM; (CC BY-NC-ND 2.0)
Havana, North Dakota

Glaciation formed countless pockmarked depressions which now serve as natural basins. Glaciers are responsible for just about everything observable here. Numerous flows of ice moving back-and-forth across the landscape left deposits that reached as high as 900 feet (275 m).

So 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 (map). 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 Wisconsin glaciation, perhaps ten or fifteen thousand years ago.


As It Appears Today

Nicollet Tower. Photo by South Dakota; (CC BY-NC-ND 2.0)
Nicollet Tower

Google Street View  (image) doesn’t do it much justice, unfortunately. Nonetheless, follow the link and 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 offer 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. North Dakota State University
Coteau des Prairies Topography from the North

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 some say Coteau des Prairies has a “flatiron” shape. The wedge and flat plateau practically jump from the screen.

Stereotypically, the North American continental interior seems bland and featureless. The landforms are not as dramatic as other areas. However, look closely enough and all sorts of interesting variations begin to appear.


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6 responses to “Coteau des Prairies”

  1. Steve Avatar

    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. Twelve Mile Circle Avatar

      Here’s the sign, Steve:

      Another image can be found on this flickr photo. It doesn’t appear to have a counterpart in South Dakota, at least according to my quick search of the Intertubes.

    2. Erica Avatar
      Erica

      My mom grew up in Summit, SD and she told me that it’s situated at the highest elevation on the Eastern side of the state. My grandpa told her the town was called Summit because it was the highest point between the Missouri and Mississippi rivers, but he’s a known jokester so I’m not sure if those things are necessarily true. Summit is located on the highest ridge of the Coteau, though, and I think it would be the first town elevated ~5,000 ft above sea level that you’d come across if you were traveling west from MA & entered SD, though it’s North of I-90. It’s funny — the elevation isn’t perceptible but the town & surrounding area are often covered in fog. They have a festival every summer called Fog Fest 🙂

  2. Former South Dakota Kid Avatar
    Former South Dakota Kid

    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. January First-of-May Avatar
      January First-of-May

      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!)

    2. SD Lifer Avatar
      SD Lifer

      Only off on elevation estimate by about 2500 feet…

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