Farm Horizons, December 2014

Unusual rock found in Dassel Township field

By Kristen Miller

One spring day, Pat Isaacson was working in the field in Dassel Township, when she came across a peculiar object in the dirt.

“I thought it was a leather football helmet,” Isaacson said, recalling the early protective head gear worn by players in the 1920s and ‘30s.

“I knew it was something other than a rock,” she noted.

It was that inclination that stopped her from discing and got her off the tractor to investigate.

She picked up the rock, which weighs about 20 pounds, and brought it home. This wasn’t going to end up on the rock pile.Rock

For four or five years – she doesn’t recall the exact year she discovered the rock – Isaacson used it for decoration around the house. She would even dress it up for Halloween and Christmas.

“I thought it could really be something, I just didn’t have an inclination to do anything,” Pat said.

Her mom, Millie Vehanen, even found the rock to be special, and would show it off.

Then one day, she thought of consulting her former high school science teacher, Aaron Moen, who recently moved back to Dassel. Moen happens to be a professor emeritus from Cornell University with a background in geology.

“I’m glad you called on me,” Moen told Pat during the interview.

“I immediately knew it was special – something worth looking into,” Moen commented.

After doing research online, Moen found that the rock Isaacson found, was indeed special, particularly to Meeker County in central Minnesota.

“I even thought, did it drop from the sky?” Isaacson thought.

Scientifically referred to as a septarian concretion, its formation is much of a mystery.

A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil, according to a description on Wikipedia.

Concretions are often spherical in shape, although irregular shapes also occur and form within layers of sedimentary strata that have already been deposited.

Septarian concretions, or septarian nodules, like the one found in Dassel, are concretions containing angular cavities or cracks called “septaria.” The word comes from the Latin word septum; “partition,” and refers to the cracks/separations in this kind of rock, according to Wikipedia.

The process that created the septaria remains a mystery. A number of mechanisms, such as dehydration of clay-rich, gel-rich, or organic-rich cores; shrinkage of the concretion’s center; expansion of gases produced by the decay of organic matter; brittle fracturing or shrinkage of the concretion interior by either earthquakes or compaction; and others, have been proposed for the formation of septaria.

It is uncertain, however, if any of these is responsible for the formation of septaria in septarian concretions.

Why in Meeker County?

Moen also had never seen anything like it before but speculates it has been left here by a glacier more than 10,000 years ago.

His intentions have been to bring it to the University of Minnesota to see if anyone there has seen a rock like this in the state.

Currently, the rock is on loan to the Dassel History Center, where it can be viewed.

Mary Jensen, museum assistant, said it’s been interesting to hear what people say when they walk by the rock.

“It piques their interest,” she said, adding that it’s a “marvelous addition” to the museum.

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