Yesterday dawned cool, sunshiny and slightly breezy–the perfect day for a 2+ hour drive and an outdoor exploration. And so I had the extreme pleasure of joining a small group of naturalists for a geology lesson offered by one of our colleagues, Denise Bluhm, at Pemaquid Point in mid-coast Maine.
The lighthouse is one of 57 still active along the Maine coast and it’s this very one that is featured on the state quarter. It was manned from 1927-1934, but has been on automatic ever since.
But that wasn’t our focal point. Instead, we’d gathered to learn about the rocky coast below. After reviewing the definition of mineral (naturally occurring solid, distinctive physical properties, e.g. cleavage, hardness, crystal form and color, and characteristic chemical formation) and rock (one or more minerals together, aka mineral soup), Denise pulled out a geologic map of Maine and pointed to our location.
She then oriented us northeastward and helped us understand that we were standing on what is known as the Bucksport formation, a deposit of sandstone and mudstone metamorphosed into a flaky shist. And then she took us through geological history, providing a refresher on plate tectonics and the story of Maine’s creation–beginning 550 million years ago when our state was just a twinkle in the eyes of creation.
Stepping below the lighthouse, Denise asked if we saw the fold. I thought I knew what she pointed to, but . . .
it took more of her insight to fully form the picture for me. The metamorphic rock, it turns out, is on its side due to intense pressure in its long-term history and thus we could examine its layers, much like the rings on a tree. That doesn’t mean I could age it, but just understand that over time various pressures and results of heating and cooling events caused the variation in color and mineral size of the bands. Lighter gray=sand and silt (composed of sea sediments), Medium gray=quartz, feldspar and biotite mica (black). Darker gray=more biotite. Greenish-gray=limy sand and silt. Rust=iron. All of it equals a gneiss (nice) or layered formation with foliation.
I think one of my favorite learnings came from the sills and dikes that show their faces throughout the rock. Sills are parallel or perpendicular intrusions while dikes run off parallel. And this particular dike featured a zigzag created by a continental collision. The Z-fold, as she referred to it, was caused by a right lateral shear. Who knew?
My understanding of the first fold Denise had pointed to began to develop more fully when she pulled out a geological compass and measured the angle of the rock to the left and then we could see the same angle on the far right and suddenly in my mind’s eye was the arc that has since eroded. With that came the new knowledge that more moons ago than my brain can comprehend, mountains reached six miles above and natural forces had eroded their 12,000-foot structures.
We crossed the rock from feature to feature, occasionally looking back at the lighthouse to note characteristics, such as the igneous dike (lighter color) that cuts across the metamorphic rock and leads to the buildings. Being made of granite, it offers a solid foundation for the tower.
And then our great leader led us to her favorite feature–the greatest fold in Maine.
Though the rocks were originally horizontal in nature, intense pressure and heat at some point in their lifetime forced such folds.
It was worth a photo call for Denise, Sharon, Judy, Karen and Penny–all sharing a brain with me for the day.
From there, we had a great view of the large granite-topped sill that is harder and thus more resistant to erosion.
As we made our way back across to it, we paused to look at quartz, feldspar and biotite mica–but lichens such as the sunburst also caught our attention.
Don’t tell Denise. But do check out those fruiting bodies–the apothecia.
Suddenly, our eyes and brains recognized the fold formation throughout.
At the huge sill, Karen posed to give a sense of height. (Ignore photo light)
And then we looked at its structure–metamorphic below and granite pegmatite with huge crystals above.
We noted swirls and imagined silly putty (invented in a barn in my hometown of North Branford, Connecticut).
And stood in awe of life.
I mentioned the large crystals–evidenced here. Far larger than the 2.5 centimeters that defines a pegmatite.
Denise showed us some popcorn migmatite and how the schist and pegmatite formed together.
One of our next stops was atop a jetty–so different in structure. This is an example of an igneous rock intrusion created deep underground.
To its side we could see a trough–known geologically as a syncline.
Through her eyes, we spotted red specks of garnet.
And began to understand the pinching and swelling from compression and shearing to the Northeast that formed sausage-shaped boudins.
Closer to the lighthouse, we noted the isoclinal folds Denise referred to as S-folds (compared to the Z-folds we’d seen earlier).
Before the day began, this was a great example of the rocky coast of Maine.
But when we climbed the lighthouse tower after our lesson, we looked below with brand new eyes and understandings (and still so much more to learn and wrap our brains around).
Thankful were we for our day spent in wonder on the rocks at Pemaquid Point with Denise.
11 thoughts on “On the Rocks at Pemaquid Point”
Love all the geology of Maine (and elsewhere) and would love to see more blogs and maybe even a GLLT lecture or 3 on this. Perhaps this brilliant guide or you could lead a geology walk for us novices. The Western Maine Mountains are full of wonderful rocks and minerals.
LikeLiked by 1 person
Yes, Sara–we did have Carl Francis, curator of the Maine Mineral and Gem Museum, lead us on a walk up Lord Hill two years ago. Sounds like a great idea to explore for next summer. I’ll add it to my list. 😉
I remember his talk very well. Unfortunately I wasn’t able to join the group for his hike. I’d love another chance to visit Lord Hill or maybe Deer Hill with a geology group walk. Thanks for considering it.
LikeLiked by 1 person
I think you passed the quiz with flying colors Leigh. Great job explaining all the geologic features and wonderful photos. Thank you for preserving it all.
LikeLiked by 1 person
Oh phew, Karen. I meant to mention the quiz. 😉 Now I’ll need to refer back to this post frequently to refresh my memory. And my notes.
On Pemiquid Point, as here in Oxford County, we really do walk on jewels.
Indeed we do, Watson 😉
Great blog Leigh! I hope your head has stopped spinning! Denise
Sent from my iPhone
Thanks Denise! I can’t wait to go back–especially with you. You gave me a totally different view.
Thank you for sharing, Leigh! I’ve been there several times, visually fascinated with the rock formations but on my next visit, I’m bringing your well written notes and observations!
Nancy Hogan! Thanks for reading my blog post and commenting. Denise put a whole different spin on Pemaquid and I can’t wait to get back there for further investigations.
Comments are closed.