Cobble Beaches: The Geology of a Worry Stone
by Julie Tennis
Broadcast 10.2021 and 3.6 & 3.9.2024

Photo by Wil Stewart, public domain.




Despite being autistic, with a good dose of social anxiety, I’ve ended up in a job where I deliver workshops to adults on a regular basis. This means being in the same room with large groups of people who don’t always want to be in that room with me. It is draining work. Fortunately, I get to do this work in some amazing natural areas, and after one exhausting workshop I wandered off to a nearby cobble beach for some alone time.

While resting on this beach covered in walnut-sized rocks, I found a silky-smooth, round, flat, black rock that fit perfectly in my palm. I carried that stone in my pocket for years, worrying it into a high gloss during dozens of workshops. It was a calming touchstone for me. Until it vanished, almost four years to the day after I originally found it. I was distraught. I tried carrying other rocks in my pocket but I’d grown accustomed to the texture and shape of this one; none other would do. I finally returned to that beach to try to find a replacement.

The beach in question is characteristic of beaches everywhere there have been glaciers: covered in well-sorted, multi-hued pebbles. There’s a rock on this beach for every personality. I passed up blue-grays, wavy greens, chert-like reds, banded browns, and cloudy agates in search of the same kind of silky black stone I’d lost. I found bits and pieces with the correct texture, but none that had the right shape. I thought maybe if I found the source material, I could find a beach with more of the right kind of pebble to choose from. But where to begin?

The rock I lost was basalt, a type of igneous, or volcanic, rock, the same kind that makes up the Hawaiian Islands. There are no basalt flows near the beach where I found that stone, so tracking down the source material wasn’t going to be as easy as I’d hoped.

During the last Ice Age, the northwestern United States, like many places in the northern hemisphere, was buried under thousands of feet of ice, over and over again. The glaciers advanced and retreated dozens of times over several million years, ending with the last retreat which began about 18,000 years ago. Each of those glacial episodes brought their own slew of material with them, leaving messy trails of rock, sand, and clay in piles of sediment called glacial till. My geologist friend, Chris, said the first step in tracing the source of my rock is to figure out which glacier deposited the till that this rock washed out of. Then I need to backtrack the path of that glacier and look for an outcrop that matches my stone. Another option could be to have my rock chemically analyzed and compared to known outcrops of basalt along the path of the glacier that might have deposited it.

During the last glacial maximum, there were ice caps in the local mountains as well as glaciers pushing down out of Canada, all of them contributing rock to this region. Talk about a melting pot! This confluence makes for gorgeous cobble beaches, but it also means that the source material could be in almost any direction, as close as a few miles or as far away as four hundred. Even with the hard work done by geologists in the last century to map different regions and outcrops of stone, there would still be reams of material to sift through to try to find a match.

I love doing research to answer questions I have about nature, but the volume of work necessary to find the source of this particular rock seemed too daunting a task. Instead, I returned to the beach again, determined to find a suitable replacement. This time I was successful and now a little flat teardrop of basalt is being polished with each workshop I teach.


Every week since 1991, Field Notes has inquired about Montana’s natural history. Field Notes are written by naturalists, students, and listeners about the puzzle-tree bark, eagle talons, woolly aphids, and giant puffballs of Western, Central and Southwestern Montana and aired weekly on Montana Public Radio.

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