This whole area has a geologic history from approx. 2.6 million years ago all the way back to a little over 500 million years ago.
The mostly sedimentary layers that were deposited over this time are made up of Shale, Limestone, and Mudstone. There is some volcanic material mixed in along the way as the age range of these deposits is so great.
Some 70 million years ago the lands around here experienced very violent earthquakes that folded, buckled and moved the various faults, some up so others sank down.
The faults I'll take you to will show you a couple examples of these movements. We the public needed roads rather than trails to get from town to town as we grew, the road cuts they made for these thoroughfares sometimes exposed some great geology lessons for us. These are two.
As you can see from the reference drawing the newer Bear Lake Plateau deposits come right up against the Bear River Range and along that zone you'll see the Bear lake fault that helped these rocks look how they look today.
You'll see how some of the older harder Limestones and Mudstones collide up on a softer Sandstone and you'll be able to see how the rocks moved when this happened. You'll also notice in this location one is Horizontal and the other almost vertical. The sandstone here also shows thick colorful layers that are very thick due to heavy mass flowing (a lot of years in a row of storms and runoff flowing into this inland sea and being deposited at a greater rate.)
In the second stop, just up the road a 1/4 mile you'll see where layers of the same type of Mudstone and Limestones reached across a fault and the rocks show a slip fault
In the last drawing you can see the different Faults and how they act with the layers, look at this close as you'll need this reference in the second location!
Logging Requirements: Please don't post pics of exact area
E-Mail me a note with:
1. The text "GC5E6CH Bear Lake Roadcut Faults"
2. How many people in your group today?
3. At the first location, looking at the layers which is the Mudstone/Limestone and which is the Sandstone? Left or right?
4. What Colors do you see of the Sandstone and how (approximately) thick are they?
5. At the second location, can you see a fault here?
6. Approximately (in inches) what is the offset of the corresponding layers?
7. What type of fault do you think this is? (see reference drawing in cache description)
The following Material were used for Reference:
- Digital Commons USU 2007 - Bear Lake Basin: History, Geology
- U.S. Department of the Interior, Professional papers and Geological surveys
- Geology.isu.edu/digital-Idaho
- Simplified geologic map of the Bear Lake area. Geological Society of America Special Paper 450
- Wyoming thrust belt province, Richard Powers
- Wikipedia