FARGO - Workers digging the foundation for downtown’s Block 9 high-rise pulled what appeared to be an enormous rock out of the ground earlier this week.

Any excavation is bound to turn up a few rocks, but this one was about the size of a skid loader.

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The Kilbourne Group, one of the developers of the 18-story building, consulted with a geologist, and the firm’s staff was excited to think that the rock might be a memento from the days of the wooly mammoth.

Adrienne Olson, a Kilbourne spokeswoman, said by email on Tuesday, Oct. 9, she heard that if that were the case, it would be the largest “dropstone” found in what used to be glacial Lake Agassiz.

A “dropstone” is an unusually large stone that’s dropped by some natural means, say being blown out of a volcano or carried by a melting iceberg, and becomes embedded in the ground.

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Donald Schwert, a North Dakota State University geologist, said by email that if it were a dropstone, it probably was scraped from the ground by a glacier somewhere in northern Canada, where these kinds of boulders are often found, and carried south thousands of years ago.

Toward the end of the ice age, when the glacier began to melt and Lake Agassiz formed over what’s now the Red River Valley, chunks of the glacier would’ve broken off to form icebergs, according to Schwert. An iceberg could have drifted along the lake with this rock embedded in it until the ice melted and dropped the rock in the lake bottom.

Schwert had planned to bring a rock pick to the construction site on the 200 block of Broadway Thursday, Oct. 11, to get a chip for analysis.

But then came the bad news.

Keith Leier, the project manager overseeing Block 9, said by email that with rain, snow and construction workers giving the boulder a wash, they concluded it was really a “large irregular ball of concrete.” What had been an exciting find has left his team “super bummed,” he said.

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The muddy lump had looked like a boulder because it didn’t look like it was a part of a building’s foundation or wall. Schwert speculated that some worker in the past had disposed of extra concrete by pouring it in a pit.

The Block 9 construction site had once been filled with hotels, dry goods stores and other commercial buildings, the first group built in the early 1880s and the second in the 1890s after the Great Fargo Fire swept through downtown.

A second fire in 1976 damaged many buildings on the block. They were torn down in the early 1980s and converted into surface parking, and it had remained that way until developers broke ground on the high-rise.

For the Kilbourne Group, which specializes in redeveloping downtown, the city’s oldest neighborhood, the boulder probably won’t be the last unusual object dug up. The firm’s previous projects uncovered the brick foundation of the long gone Carnegie library on Roberts Street and numerous forgotten heating-oil tanks.

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