Shape Shifters: Structural Folds

🚙 US-14A Mileage 6.3 GPS Coordinates N 44.24.971 W 103.53.248 4,349 ft.

Shape Shifters Update2.pdf

Lesson Guide: PRINT

It’s normal that formations lie

In horizontal form

So tilts and bends reveal to us

Some cause outside the norm

Field Exploration

Don't look now but the roadside rocks just changed their shape! It appears that solid rock is not as solid as you thought, so in this exploration we'll introduce you to the science of structural geology alongside a magnificent roadside fold. What forces produced it and why is it here? We'll ponder these questions and more in our quest for understanding this amazing feature.

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.

Panorama of Tilted Cambrian / Ordovician Deadwood Formation

Parking for this site is visible along the bottom edge of this photo. Please exercise extreme caution along the highway.

The dipping and contorted strata featured in this photo are of the Deadwood Formation. The fact that these sedimentary rock units are not lying in a horizontal position indicates that they were deformed sometime after their initial deposition in an ancient sea. In the Stratigraphic Column below, note the position of the Deadwood Formation relative to the other rock units in Spearfish Canyon.

The Deadwood Formation is the oldest sedimentary rock layer in Spearfish Canyon and predates the existence of life on the land. Its multitude of thin layers makes it susceptible to deformation. Age and thickness are approximates.

Structural elements of the monoclinal fold in the Deadwood Formation.

Structural Elements of the Monocline.

To provide insight into how a structure may have formed Geologists like to describe the structure in terms of its geometry and directional orientation. These qualities are depicted in our annotated photo and can be measured with a compass and a level. Note the position of the Deadwood Formation in the Stratigraphic column to the left.

Monoclinal fold in the Deadwood Formation.

Road-level Close-up of the Plunging Monocline.

The word monocline is derived from the words "mono" meaning one, and incline in reference to a layer's orientation with respect to a horizontal plane – a tilt or a bend. In our case the rock layers are interrupted by a single step-like fold or bend, thus mono-cline. We add the word "plunging" to denote that the entirety of the beds has a regional dip as well.

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

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