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

🚙 Located 11.1 miles up the canyon from the intersection of Colorado Bvd. and US 14A at Spearfish GPS Coordinates N 44.22.443 W 103.54.884 4,834 ft.

Site Access

A designated parking turnout is located along the outer edge of the highway directly across from the Lesson Site but because it’s surface is tilted downward away from the level grade of the highway it is sometimes difficult to see as you approach. Also, given that the turnout and Lesson Site are situated along a somewhat blind curve in the roadway it would be wise to do a drive-by first and then, once you have evaluated the situation, return to park with safety foremost in your mind. Warning: We normally we would encourage you to get as close to the Lesson Site as possible but due to the blind nature of the busy highway we would urge you to NOT cross the road but to stay with your vehicle at the parking turnout.

What's in the Icebox Update 3.pdf

In–Depth Geology Lesson pdf

A great document to print for those who want to dig deeper into the geologic story.

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 for those desiring more depth. Mileage and coordinates are provided in each document and roadmaps and other learning aids can be found at the Geologic Resources link in the sidebar menu. The document is most useful if you have it in hand at the geologic site. Exercise extreme caution at the lesson site and along roadways.

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

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!

There's more to this story than meets the eye. Download our In-Depth Lesson pdf above if you desire a more thorough coverage.

Send comments to: partial.melting@gmail.com

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