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.pdfLesson Guide: PRINT
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. Â
This pdf document contains the specific directions for reaching our featured Lesson Site as well as a detailed explanation of its geologic story. Mileage to the site as well as its specific coordinates are listed on the header above and also in the lesson document. Roadmaps and other learning aids can be found at the Lesson Resources link in the menu and are helpful to have handy in the field. Please print this document and take it with you or read it thoroughly before departing. 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.
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.Â
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.Â
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