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Superior Stones: Talkin' Thompsonite

Figure 1:

Welcome back, Bristol! Who submitted another distinguished story for Rockosophy, relating her explorations of Duluth, Minnesota. What this badass Alaskan has to say is important, not only to the wildlife biologists of the world but to the budding scientists of this fair nation, who struggle to feel out their niche in what can feel like an ever-uphill battle. Striking balance between education and work experience is a kind of rite-of-passage for many field biologists, and what we share about our individual journeys strengthens our collective representation as a work force for good science. For your meditations, musings, and entertainment, I leave you with Bristol. Be sure to check out her background on the Contributors page and her blog too (!

Figure 2: Agate sample from Lake Superior area.

"As I coasted down the highway for the final 5 hour stint of my road trip, a fervent hope crept into me. I dearly hoped my 28-hour trek to my new (however temporary) home would make up for the three days of constant highway driving. I was on my way to an internship at Hawk Ridge Bird Observatory, to investigate my interest in working with kids in nature. During my trek, the realization dawned on me that wildlife biology, while super fun and glamorous at times, wasn’t working for me. All my immediate superiors (some with Master's degrees!) were stuck working seasonal jobs; a prospect I found unappealing. The project I spent the last three years working on was basically a black hole for government funding, and I reeled at the recent mass of political statements swarming the conservation industry. To me, at least, the overall state of our field was a refutation of basic science and humanity. So, I decided the best way to work against this overhaul of the collective conservation world was to invest my talents elsewhere. My natural inclination landed me at the foot of good old environmental education. Incidentally, teaching also provides some level of stability to my life and, after months of serious consideration, also instilled me with a sense of purpose and fulfillment. Or so I hoped. So, with the conclusion of my final 2017 field season, I flew to Duluth, Minnesota, to see how I would fare translating my passion for the study of life into the moldable minds of tiny humans.

As I said, the launching point for this was my third summer working with Caspian Terns. That last month or so, the main crew wrapped up data collection and colony monitoring, and promptly dispersed. Following this mass exodus, my crew was replaced with several new additions, one of whom is the proprietor of this very blog.

Dear reader: You may have noticed she has a thing for rocks. Like, a big thing. Maybe even a huge thing. I’m going to stop before I fall further down the obligatory rabbithole of rock-based innuendo. She has that bit covered. The upshot of spending time with her is that now I like rocks, too (I have that effect on people- Rock Rat). This fits in nicely with my overall life plan of learning all the nature things.

Figure 3: Assorted pea agates, jaspers, and Thomsonite from Lake Superior area.

So, there I was, at the edge of Lake Superior, with its appealing resemblance to my first love, the Pacific Ocean. As I soon learned, the region surrounding Duluth is famed for Lake Superior Agates. A group of lovely, banded stones which come in various shades of red, orange, pink, and white [Figure 2]. Less well known, but arguably more beautiful, is Thompsonite, which is only found on the north shore of Lake Superior [Figure 1]. These stones form as sprays of mineral in pink and green with concentric bands. The high quality stuff looks like a heinously dyed dalmatian. I began my stay with a jaunt up the coast to Two Harbors, which boasts the promisingly named Agate Bay. That promise was lies. Far from the average naive tourist, I still managed to fall victim to the namesake's deceit for a day. While there are some sweet red and swirly jaspers and coal clinkers along the gravel roads and waters' edge, agates are quite sparse [Figure 3].

So. Onward. As I learned, the best and most abundant pea agates were to be found at Beaver Bay. To get there, park just after the bridge out of town, on the right side of the road, heading north. Little trails lead down to the lake shore. This is absolutely laden with tiny little agates, glowing like honey-colored gems among the rough basalt pebbles. I filled my hat with them [Figure 2]. I can only imagine that exploring the nearby river and tributaries would yield larger specimens, but that was for another day. Pea agates are nothing to sneeze at. I recommend filling a small glass container with them, and then adding water and holding it to the light [Figure 4]. There is a whole world of tiny and intricate beauty in these little shards. I did end up finding some decent sized agates, ones that fit comfortably in your hand and that you don’t need to worry about falling through a hole in your pants pocket. If you develop a rapport with local rockhounds, they might show you some good places to go, too! Alternately, I will pass on this unorthodox suggestion: Go to any random gravel road. The gravel in the region is all local, so it almost always houses pea agates.

Next, I headed to Moose Lake via a friendly recommendation. The town of Moose Lake features the Agate Museum, which is certainly worth a visit, but also hosts a dense concentration of gravel agates. I walked an east-westerly road late in the day & those babies shone like jewels in the dust. If you’re pressed for time, or have limited mobility, you can still get in some excellent opportunities to hound for quality agates. They may just need a quick bath and maybe a brief tumble.

Figure 4: Jar full of Lake Superior Agates.

Now, the whole two months that I was in Duluth, I heard tales of Thompsonite. A rare, unique stone, only found in this region, and only a few precious pieces found here or there along the shore. The rock shop in Beaver Bay features a stunning collection of Thompsonite stones, all beautifully rounded [Figure 1]. I learned from the owner’s son that the collection had taken his father 30 years to collect, primarily from road cuts four miles south of Grand Marais. He often had to cut them from the matrix rock too- carefully, lest the soft Thompsonite crack within its fierce embrace of basalt. So I was on the lookout for the stuff, but didn’t retain much hope of actually finding any.

With only a few days left of my internship, I was informed that Sugarloaf Cove was some of the best rockhounding on the lake. Although I had sternly told myself that I would use the time to pack, I thought a drive down the coast for an hour or two would be time better spent. The practicality golem in the back of my mind raved furiously at this choice, but the rest of me happily tuned her out to the blaring music of Adele and the happy anticipation of a day spent beach-combing.

Sugarloaf Cove is a natural area just south of Grand Marais that, at one time, was used to move large amounts of timber. Now it’s a charmingly forested area with some awesome geological formations. For some geological reference...Lake Superior was originally formed by a midcontinental rift a billion years ago. Instead of creating an inland sea, the continent stopped pulling apart and we were left with a massive lake and about five vertical miles of volcanic rock under my shoes. At Sugarloaf, you can observe the anatomy of those lava flows. The top layer, sometimes called pahoehoe (A hawaiian word, pronounced pah-hoy-hoy), is rounded, and it rolled shapes formed as the molten rock cooled unevenly in the open air. Under this you can find columnar basalt, like that which forms the Giant’s Causeway in Northern Ireland*. Underneath it all is a layer with tubular cavities. These formed as gas bubbled through the lava, which cooled around them. They then filled with mineral deposits over time. See Figure 5 for a nicely beach-tumbled example. All these and more are on display for the price of a short hike.

Figure 5: Beach-tumbled mineral tubules in basalt matrix.

I was fortunate to visit Sugarloaf directly after a major storm that caused all kinds of damage. The main path was strewn with detritus and halfway eroded into the lake. The beaches were tossed and tumbled as they hadn’t been all summer. Hooray! If you’re unfamiliar with rock beach-combing, going after a storm is highly desirable. New stones are tossed up and not many people have been out there recently, so it won’t be picked over. I was ecstatic.

I started, unbeknownst to me, on the wrong beach. The main beach is directly down the path, while I took a little detour through the woods. However, I didn’t know that at the time and in any case I immediately found Thompsonite. It’s fairly obvious stuff, if you know what to look for. My specimens were not gem quality, being only a white-pink color and coarsely-grained [Figure 6]. But the mineral radiated out in fibrous rounds that had a lustrous sheen to them. They broke along those radial lines, so that I found many conical pieces of broken Thompsonite. I returned triumphantly to the nature center to show my guide and mentor, Margie, my spoils.

Figure 6: Thomsonite sample.

Margie eventually redirected me to the main beach. It had really been hit hard by that storm. Trails were strewn with debris and eroded out, and a massive boulder had been pulled away from the shoreline. But the recently-tumbled beach called to me...So while Margie bemoaned the damage to the trails, my eyes were glued to the ground. I started finding rounded little shark teeth of Thompsonite, with light concentric banding [Figure 6]. Some displayed multiple radiation points, and my favorite piece also showed that pale green and pink banding characteristic of high-quality Thompsonite.


I filled a jam jar with those lovely stones, and hope to have my talented sister set one for me some day. But until then, I can happily appreciate my treasures from Lake Superior: jars of agates and agate nodules, intense jaspers, and the rare and beautiful Thompsonite [Figure 3].

From Here in the Bioshpere,


Post-script: I loved working with the kids. Teaching science is super fun and engaging for the teacher and the students. I have set myself on the correct path."

*Northern Ireland: If you are unfamiliar, please take a second to look it up. It looks like the world glitched as it loaded."

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