A rock on a section of undeveloped land on Fourth Street between Seventh and Eighth avenues east. (Photo by Matthew James)

There are a lot of rocks, large and small, strewn about Duluth. This post uses the assistance of ChatGPT, backed-up by a moderate amount of fact checking, to figure out what can and cannot be known about this particular one.

As I’ve noted in other geology-related posts, I am not a geologist. And so my starting point for understanding more about this particular rock was the starting point for so many inquiries these days: ChatGPT. With the level of confidence only an AI and the most arrogant among us can have, the machine told me the rock almost certainly came to its current location from a glacier that ripped up everything in its path, churning it through the slowly moving ice until it dropped out during the glacier’s retreat. Although this particular rock likely did not travel that far. It appears to be gabbro, a dark, crystalline rock that can be found throughout the region. And it’s not particularly round or smooth, which suggests it did not spend that long being worn down through years of glacial pressure.

While it has never likely traveled all that far, that rock is pretty old. About 1.1 billion years ago, the Keweenawan Rift, a giant crack in the earth’s crust, extended from Kansas through Michigan. Molten rocks poured out of this rift. And then around 2.58 million years ago, the Laurentide ice sheet, a vast crushing mass of ice that covered most of the northern part of the continent, appeared. It gouged out the Great Lakes through the sheer force of its size and weight. As it moved across Minnesota, it ripped up the top soil and scraped against the bedrock. As a result, in the Lake Superior region the bedrock of the Duluth Complex rock formation became exposed at the surface.

A large outcropping of gabbro bedrock on West Fifth Street, exposed when the Laurentide Ice Sheet ripped away the top soil that covered it and possibly carried a few smaller pieces down the hill. (Photo by Matthew James)

At some point, a section of this exposed gabbro bedrock broke free. It may have been cracked and loosened by thousands of freeze-thaw cycles or just knocked out of place by the sheer force of the glacial ice. However it came loose, the glacier took the boulder with it, where it tumbled and churned against other rocks as it moved along the glaciers path.

Wherever it came from, when the ice melted and receded around 20,000 years ago, that rock found itself left behind on what is now Fourth Street, between Seventh and Eighth avenues east. The houses on the lots on either side of the rock were built in 1904 and 1900. The house on the lot itself, built at the back along the alley, is actually the oldest of the three, built in 1890. In that sense, the houses were quite literally built around the rock, which makes sense as the rock has been there about 160 times longer than any of its surrounding homes.

Given the age of the rock, one might consider the significance it could have held before these homes were built, in the years before the Treaty of La Pointe in 1854, in the times when the Ojibwe, Dakota, Northern Cheyenne and other native peoples held sole sovereignty over the particular piece of land where the rock came to rest. The answer is likely that the rock did not hold much significance at all. People amongst these indigenous groups had a rather solid understanding of geology and would have recognized this as a rather ordinary rock. At least among the Dakota people, the rocks that received special attention were the erratics, large boulders carried so far by glacial forces that they did not match anything else found in the local area. A 1946 map shows the location of glacial erratics significant enough to be named, names that generally came from local indigenous groups.

Glacial Map of North America, Geological Society of America, 1945.

The rock on Fourth Street is not large enough or interesting enough to have a recognized name. But that doesn’t mean it lacks a certain significance. From its dimensions, ChatGPT estimates that it weighs about 2 tons and would require an excavator, hydraulic breaker, or possibly explosives to remove. It would get even more difficult if some portion of the rock is underground. My dad once tried to remove a rock sticking up just a bit in our backyard, just around the corner from this Fourth Street rock. It seemed about the size of a breadbox but the more we dug, the bigger our estimate of its size became. We stopped digging when it was clear it was at least the size of a car. The rock is still there, likely still damaging lawnmower blades.

This Fourth Street rock is not likely to go anywhere either and will continue to serve the one group for whom it almost certainly has had significance at different moments over the past 20,000 years. The land on which it sits has historically been home to indigenous people and immigrant groups, and is now part of a diverse multi-racial neighborhood. It would be a challenge to estimate the number of children who have attempted to climb that rock in the millennia that it has been sitting there, but I’m sure more than a few of them have given it a name. Uncovering those names is well beyond the ability of ChatGPT, but that didn’t keep me from asking. It suggested that it may have been given nicknames such as Baapaase-asin (Ojibwe for “Jumping Rock”), Den Store Stenen (Norwegian for “The Big Stone”) and Dientón (Spanish for “Big Tooth”). A complex algorithm may have made that up, but in the absence of any contradictory information, I’m going to choose to believe it is all true.

The Geoguessr games for this month looks at other glacially influenced rocks in and around Duluth. Each round lasts five minutes.

PDD Geoguessr: Duluth Rocks

More information on how to play Geoguessr can be found here.

Image note: In the featured image for this post, Adobe Lightroom’s generative remove tool was used to remove cars in the background behind the rock, as they distracted from the subject of the photo.