Geology Lesson #8, with Capt. Paul and Dr. Nat (also known as Paul and Nathalie Brandes). A trip to the U.P. isn’t complete without a visit to Mackinac Island. That’s where our resident Geologists take us today. The lead-in to the video below, comes from Capt. Paul.

“These are some photos and a video from our trip to Arch Rock. Along with a “little” description:
During the formation of the Michigan Basin, layer upon layer of sedimentary rock were deposited that formed a bowl shaped depression under present-day Lower Peninsula and eastern Upper Peninsula. One of these layers, the Devonian aged Mackinac Breccia, makes up the bedrock found on Mackinac Island. The Island itself was formed about 13,000 years ago when glacial ice carved the landscape. As the glaciers receded and melted, the resulting meltwater filled the basins to create the Great Lakes and eroded the limestone breccia making up the Island to form its steep cliffs and unique rock formations. One of the most dramatic remnants from the time of the glaciers is Arch Rock. This arch formed during the Nipissing Stage when Lake Huron was much higher than today during the end of the Wisconsin glaciation. Because of this, Arch Rocks sits 146 feet above the current shoreline of Lake Huron. The arch is also unique in that it is composed of Mackinac Breccia. Typically, limestone breccia is not an ideal material to form arches or natural bridges due to their high weathering rates, making Arch Rock a very rare sight in the Great Lakes region.”

Very interesting that the info on the plaque was correct about the age but not so much about some of the other details! Such as, it makes it sound like the arch area was always 100 feet above the lake waters. Thanks for the straight scoop Dr. Nat!

Now i'm wondering if you'll have a video on the sea stacks in the St. Ignace area!

I was hoping they visited the natural wall and
gave us a geology lesson.

Oh yes, Bob, i'd appreciate hearing the specifics on the natural wall too!

Love it! Dr. Nat, do you get paid for this? If so, where do I sign up?

Dr. Nat (& Capt. Paul)...THANKS!

Thank you, everyone! I'm glad you are enjoying these. Unfortunately, I did not film one of these at Natural Wall. Maybe next year. There are just so many interesting places to go!

Ooooooh, can we send requests for next year? ;-)

We're not positive we will be back in the U.P. next summer (research projects might require us to be elsewhere), but... we could take requests. There are so many great places to go.
And in good news, we got a new microphone so the audio will hopefully be much better!

I've been close to the natural wall and seen it from a distance, or at least parts of it through the dense forest leaves of summer. Unlike Hungarian Falls, the access is much more primitive. In order to see it properly one must climb down a VERY steep hill that encloses a deep ravine similar to those of other creeks creating Houghton Douglas Falls and Hungarian Falls, all stemming west from the Traprock river basin. I would have not made it down without falling. Never mind the impossible climb back up. If I ever go back it should be before spring leaf out or in the late Autumn. I would love to get Dr. Nat's interpretation from there someday.

Capt Paul did provide this info on the natural wall back in 2007:

By Capt. Paul (Eclogite) on Friday, February 9, 2007 - 11:03 am:

Hmmm, looks like we’ve been called out!! ;-) Since Dr. Nat is in class all day, I can give the low-down on Natural Wall. I apologize if this gets a bit long and I use some scientific jargon, but you can always ask me later…..

First and foremost, Natural Wall is not a dike. Dikes are tabular or sheet-like bodies of magma (i.e., igneous rock) that cut through and across the layering of adjacent rocks. They form when magma rises into an existing fracture, or creates a new crack by forcing its way through existing rock, and then solidifies. Once it solidifies, it’s not necessarily harder than the country rock. Natural Wall is made completely of Jacobsville Sandstone, and while it stands very near the Keweenaw Fault, there are very few basalts incorporated into the sandstone and none in the Wall. The photo NoTroll shows is a mafic basalt dike associated with the Crater Lake/Mt. Mazama region of Oregon. That is not to say that there aren’t any dikes in the UP. A dike swarm develops when the land is stretched to a point it thins and magma is allowed to come to the surface following fractures in the rock. One of these small swarms associated the Mid-continent Rift is south of L’Anse near Alberta, and other single dikes of this age can be seen along M-95 near Republic and in the Empire/Tilden Mine pits.

Now on the Wall itself. During the Mid-continent Rift stage as the land was being pulled apart (a modern day analog would be the East African Rift), normal faulting occurred along the edges of the rift. One of these major faults was the Keweenaw Fault, which runs the length of the peninsula. Towards the end of the rifting stage, sediments from the east and south (Jacobsville Sandstone) began to deposit along the southern edge of the basin that formed as a result of rifting. Next, a large continent sized chunk of crust began slamming into the east side of North America. This collision is known as the Grenville Orogen. During this event, compression on the Rift and Keweenaw Fault began changing the characteristics and movement of the Fault from being a normal to a high angle reverse fault. This compression also had a profound effect on the sandstone near the fault, tilting the once flat-lying beds of sandstone into vertical and even slightly backward. If one has the chance to walk the creek that flows beside the Wall, by all means do it!!! In about 1 km of walking the creek bed, you will go from flat to east dipping beds to vertical at the Wall. Of course as Mary said, you should obtain proper permission to enter and take a knowledgeable geo-person with you (hint hint ;-)

This is, of course, the Cliff Notes version of how Natural Wall came to be. I’m always open for more questions and a more in-depth look at the Wall, or any other geologic questions that may arise. I could yak about Lake Superior geology for hours; as you can probably tell, it’s one of the most fascinating areas on the planet, and that’s why I absolutely love studying it!!!

Good job jbuck,

A walk up the creek seems like it would probably be an easier and smaller climb but also a much longer walk. The entry we made in order to get to the top of the wall, where my picture was taken was just 100 yards off Old Colony Road going east out of Laurium. At the time in 2016 it I believe it was still private property but our group had permission.
The GPS data is:
Lat: 47.241046°
Lon: -88.396377°

D. A. (Midwested),
I knew we had talked about the Natural Wall before - had to go find the discussion. As you say, hard to see it in the Summer. Karl Berg provided a Winter picture which Mary posted on that date which is excellent.

Did I actually write that? ;-)

Thanks to all of you who have enjoyed our little "Here we are..." geology snippets across the UP. Like Nat said, if we make it back up next summer, we will surely visit more spots across the UP and the northern lower as it too has some fascinating places. What we'd really like though is to meet some of you in person, once covid is controlled of course.

Yes, yes, i think that was you Capt. Paul!

What's interesting is that back then when you explained that i as having a bit of trouble wrapping my head around it. Now after Dr. Nat's mini classes and other explanations over the years when i reread it now i think 'yes, that makes sense'!!!