What's in The Icebox: The Winnipeg (Icebox) Formation

🚙 US-14A Mileage11.1 GPS Coordinates N 44.22.443 W 103.54.884 4,834 ft.

What's in the Icebox Update2.pdf

Lesson Guide: PRINT

Of deeper water origins

This greenish shale was born

Now bears the signs of weathering

All gently-sloped and worn

Field Exploration

Strangely colored and crumbly – the remnant of an ancient sea that once covered this area. But is it really a rock? In this roadside exploration we'll introduce you to the Icebox Formation and tell the story of its response to the natural forces of weathering. Icebox . . . It sounds a bit chilly but it's a story that will warm the geologic mind.

This pdf Lesson Guide contains the specific directions for reaching our featured site as well as a detailed explanation of its geologic story. Mileage and coordinates are provided in each document and roadmaps and other learning aids can be found at the Lesson Resources link in the sidebar menu. The document is most useful if taken with you to the lesson site. Exercise extreme caution at the lesson site and along roadways.

Are you ready. . . . Let's Go! S. V. Fogarty & W. R. Stevens

*Mileage starts at the intersection Colorado Bvd. and US 14A near Spearfish.

The Icebox rock unit is actually a geologic member of the Winnipeg Formation, as is the Roughlock which is featured in another lesson stop. Age and thickness are approximates.

Google Earth 3D of Lesson Location

A parking turnout is located across from the Lesson Site along the outside edge of the highway. Exercise extreme caution at this location. We advise that you NOT cross the road due to the "blind" nature of the curve in the highway.

Icebox Formation

The Icebox Shale is one of three members of the Middle Ordovician Winnipeg Formation. See the Stratigraphic Column to the left for the position of the Icebox Shale within the stack of rock layers exposed here in Spearfish Canyon.

Icebox Shale

The Icebox was deposited in a large, inland sea that invaded the North American continent roughly 465 million ago.

Landscape Geometry

As you look up at the younger Pahasapa Limestone you'll notice that this formation stands tall in outcroppings that form vertical cliff faces. This is in stark contrast to the softer Icebox Shale which is not strong enough to do the same. Loose or weak materials like shale tend to find their stability at slopes of around 33 degrees. This general principal is useful in identifying a change of formation or even in identifying the rock types as weak or strong. Building a cabin on an outcrop of Icebox shale may not be a good idea!

Be sure to download our Onsite Geology Lesson PDF posted above for a detailed geologic investigation of this site.

Send comments to: partial.melting@gmail.com

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